It all depends on how much room you are willing to give up and what speakers your talking abut. I had a standard cab S-10 and could fit four Deep magnet 10"s or 12"s in it... of course my seat was nearly all the way foward. I did boxes with my 10's facing just about every which way possible... and if your willing to dedicate the extended cab completely to speakers you can do just about any combo. Do a few searches on boxes im sure you'll find something or you can always pay some of the box builders here to design you one.
My Favorite configuration was the one in my avatar... It hurt.
I would reccomend making a false floor to even out the back. Boxes are so much more difficult when you have to contour them to the car. I built one that way too though.