I'm not sure exactly what you mean when you say, 'I imagine that 750mA's to led would pull the voltage down considerably'. Looking at your info, you need between 2-6V of input voltage. You could run either 2x123 cells or whichever
single LiON cell fits the body... maybe the 17670 for a 2 cell body or an 18650 if it happens to have one of the 18650 compatable bodies.
To build your own, you look at it backwards... Looking at the regulator chart
here.
Basically, look at the LED you want to use "Lux V" look at the voltage range you want to send to it "8.4V" and it will tell you which board or boards would be best for you application. Looks like the Downboy is the way to go. The board sends whatever voltage it needs, but regulates the current so a DB750 will send 750ma to your Lux V.
If you replaced the light engine with a ***T Lux V on a Downboy 750, you could run your two 17500s or any two 3.7-4.2V LiON cells.