What you need is a "clamp plate" like shown here in black, but obviously with 8 lugs.
http://rides.webshots.com/photo/2817686820080251109pQiKplThey are what your truck should have had originally, and if not then they are what those wheels had in mind. I've seen plenty of them on step vans with both 16" and 19.5" wheels, and I expect on pickups too but I haven't looked close.
With the clamp plates they distribute the clamping load over a wider area than just the nut, even more still than a flanged nut. You will use tapered/beveled nuts (as if you had stud centered wheels) with the clamp plates, get the nuts from NAPA.
You can still get the clamp plates from GM, but they are ridiculously expensive, so look for used ones.
Edit: Thinking about the clamp plates again, I'm sure new ones have chamfered holes, but original ones may use flat nuts. I know this is the case with the 10 lug versions on 1.5 and 2 ton AD trucks, and I know late 8 lug ones are chamfered, guessing early ones were flat.
Grigg
