I found the manual for my '56 beast on Ebay, look around on there from time to time and you'll find one I'm sure. All of the questions you have asked are in the books and it is really better to follow proceedures for your specific year & model.
The axle you have may be listed in this
1956 GMC 100-500 series truck manual, the brakes should be pretty much the same, the adjustment proccedures are in there.
The rear end oil sounds like what was in mine, looked an awfull lot like moldy hersheys syrup with some spoiled milk in it, smelled as bad too. If you take a wire brush to the bottom of the pumkin there should be a drain plug under there, but if not I'd reccomend taking the cover off and having a good look around the rear end anyway. I found a load of pitting on the teeth of my ring gear and some of the PO's Mickey Mouse engineering that needed some attention. it's also a good way to get any residual gunk out of the rear.
