If you decide to go with the dual piston M/C, be sure and use the entire system- - - -proportioning valve, etc. The only advantage to a dual chamber M/C is redundancy- - - -in the very unlikely case of a leak at one end of the vehicle, you'll still have half a brake system, but with an EXTREMELY low pedal height. I think I'd rather keep the single M/C with a small diameter booster, one that will fit close to the original M/C position under the floor.
Jerry