The parking brake is on the output shaft of the transmission (external band type). Since the GMC CCKW used two different transfer cases (Timken and Corporation for two separate axles aka 'split' and 'banjo')
All of the Chevvy 1.5 ton 4x4s had Corporation (or banjo) while CCKW 6x6s (2.5 tons) had both depending on model/configuration.
The 5 speed is SIGNIFICANTLY larger than the 4 speed (I have a G506 1.5 ton and G508 2.5 ton) and this topic comes up every so often. Its about 30% larger and much heavier.
You would need a few custom parts (the drive shaft from the tranny to transfer) and the floor pan and will need some major work. The bell housing should match up, but the frame mounts won't. The transfer case would also need to be moved about to line it up properly.
I've never actually seen or heard of anyone doing it... but floor pans are cheap... finding a useable 5 speed is not that big a deal (I have 3, two spares). They come up on ebay... unless it has been rebuilt, they are worth their weight in scrap... $200-300. If rebuilt (and the have papers/documentation) then well over $500-800 is not out of the question. It will all depend on the rebuild.
