Rick in VA,
For GM rear ends for your 51, take a look at 67-81 Camaro/Firebird/Nova rear ends. The width is about right.
Another modern rear I have looked into is a rear end out of a Trail Blazer. Since they came out in 2002, there are many around. The width again is about right. I am finding a Trail Blazer rear end easier to locate than one out of a 70's vintage GM.
Price for the trail blazer around here is $350 at the bone yard.
S-10 4X4 rear end will also fit your truck. Stay away from the 4X2 S-10 rear....too narrow.
