i have a 12" MTX 4000 sub in my regular cab. I built the box myself, it has about 1 cubic foot of airspace which is required for that sub(they don't even make them anymore). However, the quality isn't the best because of the geometry of the box (too much length, not enough depth). I used to have two 12" subs in the back of there, but decided to just put one box that had better specs for the sub. Although i do know that Premier makes 10" and 12" subs that have a very skinny magnet that enables it to fit into small enclosures. Premier is made by Pioneer but better. If i were to ever upgrade my subs, i would definately look into the Premier subs, I'm sure they also have 8" subs, but i dont know
Check crutchfield.com for the subs, they can tell you what size and everything, you shouldn't have to even measure. Plus you can do a live chat with their tech guys and just make positive about the sub and size. Usually they have pretty good deals, but you can always buy the sub off of eBay from a respectible buyer if you want to save a little cash.
those pioneers sound like butt. I went to car toys the other night thinking about doing the stereo over in my truck and the guy tried to push them on me and i kept telling him what i had in there now sounded better and they are like flemarket 6inch speakers with a radiator in truck boxes.