As far as the disk brakes I have no idea if the newer ones will interchange. Your 63 has a solid frontend and around 65 they changed to twin I-beam. I hope someone could shed more light on that. You say it has a six assumiung it is a 300 or 240 find one or the other in the scrapyard and grab the pulleys, ac bracket and what other decent ac goodies you can find. Even if you do get all the other parts you need in decent shape do not use junkyard AC hoses. Many places that make hydraulic hose can make any hose you need for the AC. Their are plenty of sixes with ac. A C4 tranny would do ok on your truck an Fmx would do even better. imo Although they are bullet proof I would stay away from the C6 trannys unless you plan to hop the engine up a little. C6s rob a little power. Whatever transmission you get make sure you grab the crossmember. Your old crossmember will more than likely be riveted in. You can grind the rivets flush then knock them out with a punch, drill them out or very carefully torch them. Your replacement crossmember can be bolted in with grade 8 bolts. You will need a radiator with a built in transmission cooler or an external cooler. You can swap in an automatic column or go with a good floor shifter. I know I have not covered everything as I am a little tired this morning and so is my mind. Hopefully someone will have something to add.