I'd go with Hotrod on this one,,run as fasst as you can. AutoZone dont rebuild any motors, they just bid them out to place for that serice. You wont get back your orginal engine, and will have wasted what ever you paid for it. I'd pretty much guess its going to cost you around 2 grand to do a six correctly, more if wasn't a good engine to start with. Other than the braking system, the engine is the most important part of the truck. Everything else you do is just fluff around it.
