lt headers = Long Tube Headers. Better flow, especially up top.

Nelson Performance - Tuning
Thats how to order a tune. Mail Order from Allen is pretty damn accurate if your not running any big power adders.

The gears kind of depend on the route your going with the truck. If you want to cam your truck, put a turbo on, supercharge it, or just spray the hell out of it. Once you decide kind of the direction your going on, you can then pick the gears that will best fit your application. You will most likely end up with 4.10s because most applications respond better to 4.10s then any other gearing ratio.