First off Autocrossing trucks are awesome!
What class you plan to run in will determine what and how you build your truck. If you run Goodguys then it won't matter since there are no classes or rules to builds but if you plan to run SCCA autocross you will find yourself in ESP class, the only class where 6 and 8 cylinder trucks can run, and that class does not allow internal engine mods except for a small bore over for rebuilding purposes. Suspension, intake and exhaust are unlimited. You can not add boost or juice if it didn't come with it from the factory.
If you are just starting out I would go with simple mods until you really have a handle on driving the truck itself. You might be surprised to find that some mods will actually hurt you in the turns. The best place to start is by beefing up your front sway bar and go with heavy duty shocks, you want to control understeer and weight transfer as best you can. Stiffer springs speed up weight transfer but too stiff and you will pick up the inside rear tire on tight turns (and if you don't have LSD you will lose forward momentum costing you time). Get your ride height closer to terra firma so you don't roll the truck over.
Tires! R-compund 200 treadwear or lower. Tires really make the difference. I run Toyo RA1 225/50R15 on my Nissan autocross truck. Those really made the difference after my second season. I ran street tires before that and they would melt and traction (what little there was) disappeared.
Check out
www.pro-touring.com truck section for some awesome builds.
Hope some of that helps!
