The factory setup does include the dual diaphragm booster. It has a smaller diameter, but it's longer, front to back, like fatter that way, so not really "smaller".
Prop valve is different too. And you'd need a master cylinder for rear discs.
This may be something else you'd have to do for the factory conversion. Supposedly the pushrod pin on the brake pedal is in a different location on a pedal for rear discs. Someone posted this pic a few years ago showing where to move the position of the pin to:
