Procomp 7031 on 1999 GMC sierra

I know this is tacoma world, so please don't attack me haha. I have a set of 16x8 procomp 7031s and 265/75/16 duratracs that I'm in the process of selling to a guy, with as far as I know a stock 1999 GMC Sierra. I've been trying to figure out if they'd fit his truck but it's been a mix of yes and no's and I really don't want to sell him something that won't fit. So far the only thing I've discovered that might not fit is the lug nut threads. I really don't know much about rims but the Procomp are 16x8 with 4.5" backspacing and a 6x139.7. If anyone has any insight it would be greatly appreciated.

 

1500 GM trucks also use 6x5.5 bolt pattern.
His wheel studs are a M14x1.5 where as ours are M12x1.5 so the lug nuts won't work, but the wheels should have a large enough opening to accept his studs.
The center bore of the wheel is also bored to fit the toyota trucks, he will need to get hub centering rings. These can easily be purchased off Amazon.

 

So if he were to just order knew lug nuts and the hub centering rings he should be good to go? I was planning on just holding onto the cash until he could confirm they'd fit. For the centering rings, do I need to get GMC specific ones or are they universal?