If they have 6 hub cap clips, they're likely 4.5" wide. I'm pretty sure your current 6.50-16 tires weren't installed at the factory. The will fit on the narrower 4.5" rims. That's what I have on my 3100.
When GM went to the 5" wide rims (a better fit for wider tires) they reduced the number of hub cap clips to 3, but there may be 6 clip holes, and possibly someone installed more hub cap clips, but I doubt someone went to that much trouble.
