Its very hard to tell when dealing with a used engine.

If you got it with the needles still in the flush possition, i would take the head off and see if there is any scares on the sleeve or damage on the piston.

Its not too common to see a used truck come with factory settings, unless the orig owner didnt want you to know how he was running it.

I have seen so many problems with used trucks, they almost always end up spending the trucks cost over again in parts.

I would start with doing a lean out on the HSN. make sure you can get the truck to top speed and it still sounds good. If your experiance trouble tuning, i would seal it up. (this is the only other thing i can think of why the needles are still at flush). This way you can actualy tune the truck in.

JUST FYI - Its not good to run your engine too rich or too lean. So keep that in mind before you go beating on it, in its current state. to lean will cook that little thing, while too rich puts alot of extra pressure on the piston/rod/crank/ and namly bearings.

I hope this helps, let us know the outcome...
Give us some images if possible.