6.5 inch deep subs are deep man. You also have to add in the thickness for the wood you will build the box out of, the best is 3/4inch mdf. So thats another inch and a half. for a total of 8 inches and you still need the gap for airspace and all that. I would recommend you get some shallow mount pioneers and then you can try and get them behind the seat after the tank comes out. heres a link to some subs from sonicelectronix.com
http://www.sonicelectronix.com/viewc...=41&f_627=2356