I would start with a 12 inch rotor. Any bigger and you will have to grind the caliper down because it will rub in a 16 or 16.5 rim. Try the calipers they used on a 3/4 ton gm truck from the 70s. Those calipers are wider than the camaro type as the disc will be at least 1.125, and probably 1.250 inches thick. If the bleeders or hose connections don't line up or interfere with something, try the commercial chassis calipers... the ones they put on limosenes.
You should be able to use the original hub, and add a slip on rotor. The holes in the rotor are going to be much bigger than the studs on your hubs, so you will have to devise a bushing so you don't bugger up those threads. You can cut some old steel shaft lug nuts, the kind they use for mounting aluminum rims, in about 1/4 inch sections with a band saw. Thread those on to your studs, and it should hold that rotor tight enough. You might also drill and counter sink the flange of the rotor to line up with the existing holes on the hub used for locking that drum down.
Thats all I can add for now.
