Looking in my catalogs, I can't find a dual-circuit power drum/drum master in 1-1/4" for a GM truck. Looks like they went to disc brakes in front at the same time they went dual circuit in the medium trucks.
Late '70s and early '80s 3/4 ton trucks with the big rear brakes used a 1-1/4" dual master. It's a disc/drum master with large/small reservoirs, but it would work fine on drum/drum--you'd have to add residual valves in both lines, but that's no big deal. Some of these cylinders are machined for residual valves under the tube seats...I've got a few of them in my core pile.
The '70s-'80s boosters are pretty big and the mounting is sort of specific for the firewall, so the booster might be difficult to mount under the floor. If 'twere me, I believe I'd get a 10" or a dual-diaphragm 8" booster from a street rod supplier. It may be that an under-floor bracket for a light truck would fit the larger truck or be easy to modify to fit. I'm not sure about '41-46 trucks, but I know that '47 to '59 larger trucks use cylinders that have the same shape and mounting as the light trucks, with the larger bore. The earlier ones are probably the same. So the bracket might need modification to fit the frame, but the cylinder and pedal mounting would work fine.
Joe
