I've been itching to get some subwoofers in my 'whip'

and I dont want to put a crappy basstube in the truck. Is there any dimensions of a sub box that would fit in the standard cab without pushing the seat forward to much? Its been killing be that its been so hard to find a place with someone who has done it before, Any help would be greatly appreciated.
Bottom line: I need some subwoofers in my truck!
(Already have Eclipse deck, stock front speakers, pioneer 6x9's behind the seats.)
*Yeah laugh at the hubcaps!!*