Well, Audiobahn isn't the best out there, but It can sound decent. If they are going to sponsor you, It might be worth your while.
If you are going to be paying for your equipment, I would recommend 2- 12" Image Dynamics IDQ subs, and a nice Class D mono amplifier. I would recommend getting 4 ohm dual voice coil subs. wire the coils in parallel, and the subs in parallel. this will give you a 1 ohm load. A Cadence A7hc will put out over 1400 watts into 1 ohm, and the JBL1200 will put out 1200 watts into 1 or 2 ohm. Scala1 on ebay sells the JBL powervalve amps for $295 out of his store. You dont have to bid. But search for him on ebay for contact info. If you dont wanna squeeze 12's back there, go for 2-10s. get 4 ohm single voice coils. Wire them in parallel for a 2 ohm load, and run a cadence A5HC, or a JBL600.1. For good imaging lose the 4x6's and the rear speakers. Get a nice component set, and install them in the doors or make kick panels. Give them 100 watts, and your truck will jam.
12's will fit. Here's one I did a while ago.z-71 system