55 with mustang II, 2 in drop spindles, haven't chosen width of rear axle yet. gonna put no limit fatbar 4 link. Should I pick wheels first, then build a rear end? or get a rear end and pick wheels that fit....

 

The set up is important. Most of this has to do with how wide a tire you want in the back. Do you want to widen rear fenders, mini-tub or just a stock look. So I would figure out wheels size first.

 

Jrock,

I have basically the same setup on my 55. I am running a 9 inch from a 71 Mustang with 28 spline axles. If I remember correctly the end to end flange length of the housing was 57". I then added disc brakes to it. I am running 15x9 Prostars with 28x12.50x15 Mickey Thompsons. My wheels have 5.5" of backspacing. I also added some cut down C-10 bed tubs and recessed the bed wood for plenty of room. If you want to look in my garage I believe I have some pics of the setup in there.