Friend of mine has a '83 GMC Vandura. It's a 3/4 ton and has 8 lug axles on it. He needs to be able to put 5x5.5 wheels on it. The reason I say this is the wheels he needs to run only came in 15" rims with a 5x5.5.
So...I'm sure I could do wheel adapters, but 8 to 5 lug is going to mean 2 piece, any they will probably stick out to much.
My next question is how hard would it be to swap axles? I'm guessing it's a 14 bolt or something in the rear and I have no idea what the front end is like in the GM's. I'm guessing some kind of IFS.
any help would be great, as GM is not my specialty
Don't do it! A 15" wheel w/ that small bolt pattern is not meant for a vehicle heavy enough to have come w/ an 8-lug 14 bolt. What I mean is that I'm sure those wheels won't be strong enough to support that vehicle. It probably weighs between 5,000-6,000LBS.
An '83 is probably a semi-floating 14-bolt rear (9.5" RG) (You need to remove the tire to remove the axle shaft.) You would need to have a set of custom axle shafts made, and even then you still have problems. I'm not even sure if an axle company would be willing to do it, since it creates a safety issue. I'm also not even sure if you can fit such a small bolt pattern on the face/flange of that axle. Also, the "stub" on the axle face, which locates the wheel, would need to be machined down, or the wheel center opened up; a bad idea. Additionally, the brake drums are probably waaayyy too big to fit inside a 15" wheel.
Now, if it has a full-floating 14-bolt(10.5" RG) (Big hub in the middle of the wheel, with a removable cover- you can remove the axle shaft w/o removing the wheel), then you've got a stronger, better axle, but you would need a custom hub, not an axle. Again, I'm not sure if it even possible; and it is certainly ill advised. Also, a full-float 14-bolt will have even bigger brakes.
Also, beyond the strength/load rating of the wheel, it's just a bad idea to put such a small wheel under a top-heavy, and just plain heavy vehicle like a G20 van. That vehicle is designed for 16" 8-lug wheels for a reason. (Ok, technically they came w/ 16.5" stock, but those are hard to find these days.) There are plenty of 16" heavy duty truck rim options these days. Everything from fairly lightweight "fancy" aluminum stuff, to super heavy-duty, bead lock equipped steel rims, and everything in between. FYI, the bolt pattern is 8x6.5
http://www.pirate4x4.com/tech/billavista/14b_bible/14b.pdf
Approx 8 pages down, is a side-by-side drawing of the semi and full float versions. (semi-float, which what you probably have, is the one labeled M1009)
This guy has a ton of great tech writeups. The one on brakes is just mind-blowing. Just be prepared for some hard-core, brain melting education!
http://www.pirate4x4.com/tech/
The front of the van, assuming it is 2wd, is a simple UCA/LCA design w/ a coil spring in the middle. Basically the same design as many older rwd cars, just bigger.