Some back-of-the-envelope calculations: your power, assuming you are connecting the resistor between 600V and ground, is 818W. This means your average heat flux is about 58 W/cm2 (the peak flux will be higher, since the packaging and mounting areas of the resistor will not be producing heat), which is on par with a computer processor, but the total amount of heat you're dissipating is much higher.
I think the problem you're going to run into is getting heat away from the resistor and into the dissipating areas of a heatsink. The derating curve for your resistor starts to roll off at ~130C so you're looking at a maximum temperature rise of about 100C, which means that your heatsink needs to have a maximum thermal resistance of 0.12 C/W. On top of that, you need to deal with their ambiguous datasheet. They mention a maximum "operating temperature" of 175C. Is that internal element temperature? Baseplate temperature? This Riedon datasheet gives a maximum flange temperature and the thermal resistance between the flange and heatsink of 0.1 C/W which seems high; similar resistors have thermal resistances of 0.025 C/W, which means that your heatsink needs to be lower than 0.095 C/W at the very least (and if your thermal resistance to the heatsink is 0.1 C/W, it needs to be better than 0.02 C/W).
0.095 C/W in air is certainly achievable; a Wakefield-Vette 512-12M heatsink can do 0.045 C/W in a 100 lfm stream and if you really blast it, I'm sure you could do better. But like I said, getting that heat out of a 188mm long package and into a 300mm long heatsink is the difficult part; you'll have a thermal gradient from the center to the ends, which will reduce the effectiveness of the heatsink and you're already on the edge. You could machine grooves into the base and press-fit some flat heat pipes in there, but I'm not bullish on that.
Where does that leave you? Going back to the Riedon datasheet, you'll see a note further down that states "liquid cooling highly recommended". 818W is about 2800 Btu/hr. A modest-sized (230mm square) liquid-air heat exchanger can deal with that with room to spare. It will cost a pretty penny but it's your best option. You could also, now that I think about it, split up the resistance across several smaller resistors that you could then easily cool with smaller heatsinks. If the cost of multiple resistors was a factor before, knowing how much it will cost to cool a single resistor and how much work it will take might tip the scales.
