I would not buy rims until I had the rear end shortened and mounted permentaly. Are you going to use ladder bars, four link or leaf springs for the rear suspension? If leaf springs are you moving them into the frame or leaving them in the stock location? I have a 1969 Dart GTS that I raced for several years, I had the leaf springs move into the rear sub frames and used a set of 8.5 Centerline wheels with 4 7/8 offset with the stock width OEM A body 8 3/4 rear end. That car had the stock rear wheel tubs in it and had plenty of room from 29.5x105 x15 slicks on the inside edges to the stock wheel tubs and frame rails