Going with the Camaro/Firebird 2nd or 3rd generation diffs, along with the 5 lug disc conversion on the front, saves you from having to spring for the $100+ special vendor roller bearing sets for the front and lets you use a wheel bearing/rotor/hub that's readily available from your local parts house, but you wind up needing the 4" backspacing.
I liked the idea of getting a diff which was readily available, with lower mileage, and being able to get replacement parts locally, without having to wait along the side of the road for a 5-7 working day delivery from UPS.
In your case you might be better off to scout up an open drive diff from a later model pickup and be able to use some available 6 lug rims. Some of the other guys who have done this swap might be able to offer some better suggestions. My experience is limited to what I had to do on my 54 to get her operational.
Dave
