I have an extended cab but I wanted to save space for cargo and passengers, so my case is similar.
I'd suggest either a 5 channel amp (which is what I have done) under one of the seats powering both sub and speakers.
Or a mono amp under the driver's seat connected to a truck enclosured subwoofer, and a 2 channel or 4 channel amp under the passengers seat hooked up to some up-market speakers.
If you want to go the cheap way you can still make a good system out of Wal-Mart's Sony selection. Get the 5x8 speakers, the 10 inch subwoofer, a slim encloser for behind your seat, and a slim sized amp for power. Head unit doesn't make too much of a difference as the amp is what is going to feed the power, but find one that has the features to price content you like.
Good luck, feel free to ask questions