Grigg, your thread has a lot of useful data in it. Thanks for sharing. It's going to take some time to wade through.
According to my calculations, width is going to need be 64.25" to 65.25". That is with 16 x 6 GM wheels wearing 7.50-16 tires. That centers 9" of tire in the 12" wheel well. One thing that I am looking for is a listing of widths. With the popularity of the 14 bolt I believe such a page exists.'