Wedge brakes have always been a problem. Every piece of equipment I ever bought with wedge brakes, I have converted to cam brakes. Figure out what rear you have, and buy a banjo housing with cam brakes and use your third member. Cam brakes are not necessarily more modern. Cam brakes were used back in the 30's. Wedge brakes came out in the 70's for weight savings. It was mostly LTL carriers that specced them because they looked good on paper, but were a failure in the real world. I think the last Gunite number was 2658G.

http://www.accuridewheelendsolution...PartDisplayBRAKE_DRUMS&PartNum=2658A

I had a pallet of them laying around until scrap got so high in 2011. You might call a Wabash National Trailer dealer. They are big with Gunite drums. The numbers off of the spokes won't do any good. They are merely casting numbers for for the wheels. But my recommendation would be to find a banjo housing with cam brakes. I would assume you have an 18.5 Eaton Rear. In the 70's GMC's big market was fleets which explains the Stopmaster brakes. The rear end hosing doesn't necessarily have to come off of GMC.