It really doesn't matter how you write the code, it will be synthesized to the same thing. A 136 bit 2:1 mux is really not that bad. It's really the number of inputs that really dictates the complexity and causes timing issues, not so much the width, though that does place a large fanout on the select signal. If that was a 2 bit 136:1 mux, then maybe you could run in to issues. I have a design with lots of 256 bit wide muxes and it works just fine at 250 MHz. Also, the tools could the muxes that get inferred there and combine them with downstream logic.
One thing that you might want to take a look at, though, is where that mux ends up in the logic and where the select line is coming from. If the select signal is the result of a large, complex operation and the mux is directly feeding a lot of complex logic, then you could run in to timing issues unless you move the mux further down that logic and/or add a register on the select line.