You can machine the center hole to get any of the 8 x 6.5 wheels to bolt up, the bigger concerns are the offset and to a lesser degree the load capacity of the wheels. info I posed on another thread -
After a little research I think I found the stock specs from GT wheels. 16.5 x 8.5 with a +50mm offset and 6.7" of back spacing. For those that know wheels, that is a pretty high positive number. Something you typically find with a front wheel drive set up. Considering that most GM trucks that use the 8 on 6.5 don't have them set that far in, I am concerned with the aftermarket world since that is what most of the wheels are built for.
The most back spacing I have found on a 8 on 6.5 is around 5.75 on a 9" wide wheel. This would push the outer lip about an inch and a half (1/2 inch of increased wheel width + 1 inch of lost back spacing). This increases the "swing" of the tires making it more likely to rub while at full turn. Not sure how much 'extra' room there is in the wheel well. Just hate the idea of wrapping up a bunch of money into wheels that I can't use about as much as I hate the idea of laying out $400 a piece for GTs that have the correct offset.