I've used the 3rd generation (specifically 88 and 89) Camaro diffs in my 54 truck. They are 62" wide, mounting surface to mounting surface, are very common in 3.08 as well as 3.42 and in addition, available for about $100.00. You'll have to scrub off the coil spring mounting hardware and set up your new spring perches. (tractor supply, trailer dept), extended u-bolts from Azone or Advanced.
I've got pics of the process in my 2 Chevy albums.
Dave
http://community.webshots.com/user/Fla54Chevy3100
