My book says between $5000 and $12000, that's the jump from one condtion to the next. If you think you can show it and win trophies, then I'd lean closer to the high side. I doubt you're going to get it for $4500 on an $8500 asking, though, not everyone selling something is desperate and starving.
If it is a solid truck and totally original, I don't see how $8500 is too much. If your plan is to sell it for more, you probably can't, but if you plan to keep it and enjoy it for a few years....go for it. Good, solid, original vehicles are not easy to find. Around here $5000 gets you a rusty bastardized hulk.