You could use MB rear wheel cylinder on a GP rear brake if necessary. The GP rear (and front?) both used 7/8" pistons, while the MB rear uses a 3/4 " piston. The brake lines hook up the same for MB and GP rear brakes. The tricky part is GP front brakes because the correct GP front wheel cylinders are cast so the banjo bolt that holds the end of the brake hose fitting onto the wheel cylinder, enters the wheel cylinder on a slite angle so it can be screwed in or out without hitting the steering knuckle casting by the upper king pin bearings. Because it uses a banjo bolt to pass fluid into the wheel cylinder, there is no tapered brass insert in the bore like an MB cylinder since the banjo bolt does not need to bottom out like the MB "S" or pig tail brake lines do (the S lines screw in and seal by compression against the brass taper insert). The banjo bolt seals by copper gaskets under the head of the bolt and the surface between the brass fitting (which is screwed to the end of the same front brake hose as the MB, and sealed by its own copper gasket), and the wheel cylinder itself. This means that the mating surface on the wheel cylinder also needs a machined face to seal properly. If you try to use an MB front cylinder on a GP, some MB aftermarket cylinders may not have this face machined and you will never seal using the banjo bolt set up. Also since the MB cylinder is not bored on a slite angle, if everything is put together on the axle and you now try to screw in the banjo bolt, it will not go in due to interference with the king pin bearing (see above). Remember that the MB brake set up has the hose ending at a special bracket on each side of the front axle, the brake hose ends there and is held to the bracket with a U clip. The final connection to the wheel cylinder is accomplished by the S line or pig tail line and as the wheel turns the pig tail turns in the same arc so there is no binding. The GP front hose with special fitting for banjo bolt hook up comes into the wheel assy from the back not over the top like MB. The hose is flexible enough to move thru the turn of the front wheels BUT does not necessarily travel in exactly the same arc, it needs to stay flexible. I believe on second series GP's there was a clamp put on the hose and this clamp had a spring attached which went to the frame (?) so the hose wouldn't sway downward because of no support. I never saw these on any GP I owned, but that doesn't mean they weren't used, though any GPer out there may have these on their restored GP. (Again excuse my inexperience with the springs on brake hoses, they may have been used just on the 1 rear hose, but again I have never encountered this myself). It is possible the military got away from the banjo bolt hook up on the jeep because from the end of hose thru banjo bolt to wheel cylinder required 3 copper gaskets (I believe of different size) while the MB hook up required just the S or pig tail line with compression style fittings at each connection point.
Some people may have used MB front brake hook ups on their GP possibly, but since the GP axle doesn't have the bracket on top of the steering knuckle, the assembly can flop all over the place. Now a guy could probably mount these MB brackets on his GP, but then how close to correct do you want to be.
Adison, you have my phone # if you'ld like to discuss this further.
By the way, I believe the Bantam also used the same front cylinders as the GP. But a Bantam guy would know for sure.
Best wishes; Leo