Don't know what the steering linkage on your GMC will allow. On my 52 Chevy 3/4 ton I found some GM 8 lug 16X6 rims. On the rear I have these with 7.50-16 radials, Set back is deeper, had about 1" clearance on the inside. Now have a HO52 from a 69 (done for better brakes) and the tires set out a tad farther, leaves 1" clearance to the outer fender loaded. Haven't gotten rid of the lock ring 15" rims on the front, with the tie rod ends clearance is slim and doesn't allow the rims without major changes. Trying to find a solution. If your GMC has the tie rod ahead of the axle the rims might work, but I'm not sure. An easier score is the GM 16X8 rim. You can also use Ford or Dodge rims, but the center hole is larger, which reduces the load capacity.
Another option is a company in California that will take the centers from your rims and mount them to some newer rims. I've forgotten the name. They can also change the setback to better fit your needs, but you need to do some measuring to find what y'all need.
