use seat risers and you can squeeze two nice ported 12's underneath. IMO, I would NOT cut THAT truck up. maybe if you had a ram or something, but NOT that SS.
If you really wanna have fun you could do what I did. (Look at my pics in my Gallery) My rear seat has been removed and the pins ground down. I have a metal frame for my amp rack welded to the body of the truck and the subs are in the bed in a square box with a blow thru. I love it. The bass sounds clean and hits hard. If you don't mind losing the space then it works great.