I bought an aftermarket kit with spindles, calipers, splash shields, rotors etc, had the same problem the attaching hardware were not correct. I had trouble with the calipers and pads as well. Eventually I just used the new rotors and put the rest of old GM stuff back on after refreshing it.
Some time in the past the shifter cut out in my car had been molested. I didn't know how to weld back then, but I do know about riveting aircraft skins, I make an aluminium layover patch to cover the hole.