Companies such as Master Power Brakes, and many other hot rod part companies make universal under-floor pedal assemblies that should work fine for your application.
I would recommend power assist for 4 wheel disc brakes. The larger the diameter of the booster, the more assist you will get from it. If space is limited under the floor, you could use a dual diapraham booster unit. They come in 7" and 8" diameters and they work very well.
A 1" bore master cylinder for disc brakes would probably be a good bet for your application.