I have a 2000 k/2500 (older square body style) extended cab. If you remove the rear seat there is the next question of if you want a wall or a below window line type of enclosure. If you are willing to walk it off the sky is the limit. But, if you would like to keep the build below the window line you will be limiting your space options.
I'm currently about to build dual 12" box for my truck and have several 12's lined up to try out to see what I like best. Depending on your budget I'd suggest looking at some sundown x,zv4/5, or even some wardens or psi platform subs if you'd rather go that route. Lots of people here can help chime in on their favorite brands of drivers, but my opinion would be to hammer down what kind of space you are willing to sacrifice first..