You may also check Ohio Generator for alternators. I personally had a Powerline in my SQ Nissan P-up. It's a small case GM that took some custom brackets but it worked. I figured it I can build the install that I did then brackets should be a piece of cake. They were.

As bumpin said, unless you do something WAY out there (like making your truck a compression chamber ala Alma) it's going to be very difficult to hit BIG consistant numbers on any meter with a "street legal" system. You COULD do a "blow through" into the cab with a cover on the bed. I've done respectable numbers going that route with a bandpass box. (mid 50's with roughly 2000 actual watts and 4 15's)

Also as Yota said, you need to decide what you want to do (full on competition, street legal, daily driver) before you build. I can't speak for dB Drag, Meca or anyone but USACi but we have classes that pretty much let anyone play against others with like systems. I plan to post the rulebook online this weekend so stop by http://www.soundoff.org and check it out. If you have any questions, drop me a line at tim@soundoff.org and I'll help as much as I can. It sounds like bumpin is also pretty "strung out" on SPL and audio in general so he is going to be a great resource also.