6 on 5.5 is the bolt pattern. The issue becomes one in which the wheel is hub-centric or lug-centric, which means does the hub do the centering of the wheel or do the wheel lugs/nuts do it? If the wheel is hub-centric, you have to be sure that the wheel will mount to your hubs. There should be a buncha threads on here that people have posted to about which wheels will fit.
If nothing else, go to
www.tirerack.com and play with the site and see what they say will fit.
If you looked at
www.summitracing.com at some wheels, you'll find that they will state "pre-71 truck", for the 6-lug wheels that will fit.