Nope, the 39 GMC actually used the wider 40-46 bed. Rims are 7" with 4" backspacing on the front and 5" backspacing on the back.. Rear tires are 235 60 15 and the fronts are 205 60 15.

Heres a new pic with the new rims and tires.

http://hotrodders.com/gallery/data/500/medium/IMGP0249.JPG