Well the stock width of a 57 3100 rear is about 62 or 62 1/4 inches. That's a good place to start. What lug pattern do you want to run... 5 on 5.5(f*rd truck), 6 on 5.5 (chevy truck) 5 on 5, 5 on 4 3/4, 5 on 4 1/2...?
How about gears and axle splines?
The 9 inch is a good rear axle with plenty of aftermarket upgrades. It may be cheaper to order a new 9 inch to your specs, rather than rebuilding a wrecking yard one. Check out
http://9inchfloater.com/. This company can outfit you with a new nine inch at any width, with any lug pattern, and disc brakes. I spec'd out a 61 inch wide, 31 spline, 6 lug on 5.5 bolt circle rear with 12 inch by 1.25 inch rotors for my 1940 gmc. This unit is much stronger than a regular 9 inch. It seems to me that most wecking yards want at least 3 bills for a rearend that needs complete overhaul. That $300 rear end will cost you $1300 before you have it installed.
