I've upgraded my brakes on my GTB, both front and rear.
I kept the same calipers but fitted larger discs and spacers - the difference in braking is incredible
At the front I have 315*28 8 groove discs with EBC green pads and the rear has 312*20 with 8 grooves and standard pads. I also have braided hoses all round.
Initialy I just upgraded the front, but the car feels more balanced under heavy braking now.
All discs have alloy bells, with the rears having the drums built into the alloy bells.
I would have thought it would be difficult to fit the front calipers to the back, as you would need discs of a particular size and special spacers to allow he larger pads to fit a disc that also fits around the drums
Pictures of brakes on website