Your tire guy mis-measured the bolt pattern, it's 6 x 5.5" on the 6 lug trucks. Late model 6 lug wheels from full size trucks do have the same bolt pattern, but the backspace and offset will not allow them to be used on the older trucks without interference issues with the suspension/steering. Same problem occurs when using import truck 6 lug wheels. You'll want to find wheels that are a fitment for '87 and earlier 6 lug GM full size trucks, wheels for the '71-'87 1/2 ton 4x4 work perfectly.
