Hi Kirbot. Sounds like you are going to build a pretty sweet light. A buck driver will automatically provide the appropriate voltage as long as it is supplied a higher voltage and the voltage supplied is within its operating limits.
The MT-G2 is one tough hombre. I would recommend you go with either the HX-1175b or the "3T6" type driver. Both me easily modded to provide as much current as you would rationally want to. The HX-1175b is 32mm in diameter and is the better of the 2. Here is a thread on it.
The original 3T6 driver no longer seems available. Not sure if there are any threads on the below driver. It is 26mm in diameter.