you could submit the vin to the gm heritage center and they will tell you what was stock on it. they are pretty quick with a response.
http://www.gmmediaarchive.com/
or this
vehicleinvoice@gmmediaarchive.com
either way, I would pull the drums and check out everything in there to ensure you get what you need the first time. like said earlier, if you check the local parts places at least you have a sample to compare. also, if it was possibly retrofitted with duallies, insure to check the stud sizes to ensure a set of single wheels will fit over. I think the actual duallie trucks had a different hub set up with less studs. don't quote me though.