• Unless an actual pre-test shows differently, do not use a cheap&poor thermal paste here (+70W). There is no need to be the fanciest paste, but for such power levels, avoid the cheapest paste with unknown thermal properties.
• Use a thin layer: t ~= 0.05mm and “mind the gap”. Use just enough paste and most important: close/reduce the gap by tightening the bolt/cramping method accordingly. Avoid scratches, as even a small one can raise marks and increase the filling gap.
• Heatsink in “Turbo mode” - Heatsink+fan dissipation capacity can be “overclocked” (as the the CPU) if you apply a controlled overvoltage to the fan.
  Most fans use brushless motors operated at 12V Nominal and accept being 15V overdriven (max) without major problems - apart from increased acoustic noise level.
  I would not recommend going higher than +25% (V>15V). Even better if this “turbo mode” is used only if Temperature becomes “Too hot”.

• Unless an actual pre-test shows differently, do not use a cheap&poor thermal paste here (+70W). There is no need to be the fanciest paste, but for such power levels, avoid the cheapest paste with unknown thermal properties.
• Use a thin layer: t ~= 0.05mm and “mind the gap”. Use just enough paste and most important: close/reduce the gap by tightening the bolt/cramping method accordingly. Avoid scratches, as even a small one can raise marks and increase the filling gap.
• Heatsink in “Turbo mode” - Heatsink+fan dissipation capacity can be “overclocked” (as the CPU) if you apply a controlled overvoltage to the fan.
  Most fans use brushless motors operated at 12V Nominal and accept being 15V overdriven (max) without major problems - apart from increased acoustic noise level.
  I don’t recommend going higher than +25% (V<15V). Even better if this “turbo mode” is used only if Temperature becomes “Too hot”. So, if that 72W max is of intermittent/sporadic nature, this “turbo mode” may be enough even with less expensive or repurposed parts.

• The TO-based transistor needs to be connected by wiring soldered on transistor’s terminals. Connectors or direct mounting on PCB are not advised IMO.
• To maximize heat transfer, the hole for the bolt should be positioned won’t in the center, but with an offset, to leave the heat-generating part of the transistor in the center of the heat-sink plate. The exit path of transistor-wiring influence the orientation/rotation.
• Hole can be made to allow internal thread - then you don’t need a nut on the back (which would be practically infeasible).
• After hole & thread are done, lightly countersink the hole - 0.5mm is enough.

Q1 & Preliminary TO-based transistor fixing to heatsink:
You can simply press/squeeze the TO-220 ou TO-3P or TO-247 against the heatsink.
Try using a metal (aluminum, steel) bar (retangular or L-shaped) and search your bolting box for self-threaded bolts (as those used for MDF/wood working) and (temporarily) tighten/clamp using two of those diagonal pass-through holes.
If testing seems good, then improve this press-mounting method to make it permanently adequate or go to the drill&thread&countersink method.

Q2 & Heatsink suitability
Pre-sizing the heatsink: Check to which CPU that Heatsink was designed for. Then search for its TPD (total power dissipation). For instance, an Intel Core i7-7700K, has TPD ~= 90W and the CPU s continues to operate accordingly - the temperature does not heat up so much (T>100°C) and maintains the CPU working right.

When you don’t have Specs: In this case, I would recommend you do a simple test. Temporarily mount any TO-based BJT (as TIP122) in the heatsink (see above) - use thermal paste. Make the power transistor dissipate 10W (Vce = 10V x 1A, or equivalent), while running the cooling fan at known voltage/speed. Measure the temperature rise (before x after 10 min).
Calculate the equivalent Thermal resistance:°C/W (Celsius degree/Watt).
With this method, you will definitively know
if this unknown Heatsink+fan will work for you.

Now, conservatively assuming the cheapest thermal paste (boxed in yellow), with a low-force pressing resulting in a (thick) layer with these following values:
Q = 70W.
t = 0.1mm = 1x10^-4 (m).
kc = 0.4 W/mK.
A = 15mm x 20mm = 3x10^-4 (m^2).
dT = (70 x 1x10^-4) / (0.4 x 3x10^-4) = 58°C

Conclusions about thermal paste:
Poor paste: In yellow - summing 61°C (transistor) and 58°C (paste), at ambient = 25°C a this leaves just 6°C for the heatsink itself - for 70W, this gives a performance figure of 0.09 °C/W. Not feasible with poor paste.
Good paste: In blue - it is ten times better and has kc = 4 W/mK, dT = 6°C. Thermal margin is (150 - (61+6+25)) = 58°C. Then Heatsink = 0.83 °C/W. Feasible heatsink+fan+good_paste, but test if yours is good enough.
Some reminders:

1. Unless an actual pre-test shows differently, do not use here (+70W) a cheap&poor thermal paste. There is no need to be the fanciest paste, but for such power levels, avoid the cheapest paste with unknown thermal properties.
2. Use a thin layer: t ~= 0.05mm and “mind the gap”. Use just enough paste and most important: close/reduce the gap by tightening the bolt/cramping method accordingly. Avoid scratches, as even a small one can raise marks and increase the filling gap.

Closing notes:
Even for a DIY project, try to do some reduced-power tests - a “pre-test” with your heatsink+fan and a similar-sized transistor, but using a lower power setting - as 10W. This can avoid bigger frustrations and problems when your project is completed.

• The TO-based transistor needs to be connected by wiring soldered on transistor’s terminals. Connectors or direct mounting on PCB are not advised IMO.
• To maximize heat transfer, the hole for the bolt should not be positioned in the center, but with an offset, to leave the heat-generating part of the transistor in the center of the heat-sink plate. The exit path of transistor-wiring is influenced by the orientation/rotation.
• Hole properly sized would accept an internal thread - then you don’t need a nut on the back (which would be practically infeasible).
• After hole & thread are done, lightly countersink the hole - 0.5mm is enough.

Q1 & Preliminary TO-based transistor fixing to heatsink:
You can simply press/clamp/compress the TO-220 ou TO-3P or TO-247 against the heatsink.
Try using a metal (aluminum, steel) bar (retangular or L-shaped) and search your bolting box for self-threaded bolts (as those used for MDF/wood working) and (temporarily) tighten/clamp using two of those diagonal pass-through holes.
If testing seems good, then improve this press-mounting method to make it permanently adequate or go to the drill&thread&countersink method.

Q2 & Heatsink suitability
Pre-sizing the heatsink: Check to which CPU that Heatsink was designed for. Then search for its TPD (total power dissipation). For instance, an Intel Core i7-7700K, has TPD ~= 90W and the CPU continues to operate accordingly with the correct CPU-heatsink model - the temperature does not heat up so much (T>100°C) and maintains the CPU working right.

When you don’t have Specs: In this case, I strongly recommend you doing a simple test. Temporarily mount any TO-based BJT (as TIP122) in the heatsink (see above) - use thermal paste. Make the power transistor dissipate 10W (Vce = 10V x 1A, or equivalent), while running the cooling fan at known voltage/speed. Measure the temperature rise (before x after 10 min) in the heatsink surface close to the transistor.
Calculate the equivalent Thermal resistance:°C/W (Celsius degree/Watt).
With this method, you will definitively know
if this unknown Heatsink+fan will work for you.

Now, conservatively assuming the cheapest thermal paste (boxed in yellow), with a low-force pressing resulting in a (thick) layer with these following values:
Q = 70W.
t = 0.1mm = 1x10^-4 (m).
kc = 0.4 W/(m.K) (wrongly marked as “W/mK”).
A = 15mm x 20mm = 3x10^-4 (m^2).
dT = (70 x 1x10^-4) / (0.4 x 3x10^-4) = 58°C

Conclusions about thermal paste:

• Poor paste: In yellow - summing 61°C (transistor) and 58°C (paste), at ambient = 25°C a this leaves just 6°C for the heatsink itself - for 70W, this gives a performance figure of 0.09 °C/W. Not feasible with poor paste.
• Good paste: In blue - it is ten times better and has kc = 4 W/mK, dT = 6°C. Thermal margin is (150 - (61+6+25)) = 58°C. Then Heatsink = 0.83 °C/W. Feasible heatsink+fan +good_paste, but test if yours is good enough.

Some tips:

• Unless an actual pre-test shows differently, do not use a cheap&poor thermal paste here (+70W). There is no need to be the fanciest paste, but for such power levels, avoid the cheapest paste with unknown thermal properties.
• Use a thin layer: t ~= 0.05mm and “mind the gap”. Use just enough paste and most important: close/reduce the gap by tightening the bolt/cramping method accordingly. Avoid scratches, as even a small one can raise marks and increase the filling gap.
• Heatsink in “Turbo mode” - Heatsink+fan dissipation capacity can be “overclocked” (as the the CPU) if you apply a controlled overvoltage to the fan.
  Most fans use brushless motors operated at 12V Nominal and accept being 15V overdriven (max) without major problems - apart from increased acoustic noise level.
  I would not recommend going higher than +25% (V>15V). Even better if this “turbo mode” is used only if Temperature becomes “Too hot”.

Closing notes:
Even for a DIY project, try to do some reduced-power tests - a “pre-test” with your heatsink+fan and a similar-sized transistor, but using a lower power setting - as 10W.
This will avoid bigger frustrations and problems when your project is completed.