Any iron diff from 63-79 will work. 63-68 housings are tighter internally and use different shims that should be ground to fit.
The 69-79 housing are wider and use the shims in the master kits. The external holes are the same,except on the 63-64 the front bracket hole is tapped and the later holes are drilled through for a bolt.
Your 64 can be rebuilt better then new providing an Eaton posi is used and the rest of the parts are high quality.I've some cheap rebuilds out there.
The 65-68 Eaton posi's are known for cracking so you must check them close. The spiders used in the 65-70 are 10-18's and not as strong as the later 71-79 10-17 spiders. The clutches in the 65-70 are solid steels while the 71-79's used the much weaker snowflakes.
Your best bet is to decide on how you want it built; "plain jane" stock or modified and blueprinted for better performance and life. Then decide on a ratio and go from there. I've built plenty of 63-64's but never one in stock form, all had tuned and polished posi's with new gears.
