Two 10s
 

I have to Infinity Perfect 10s that require 6.5in of mounting depth. I have a 2004 F150 Supercrew. Any suggestions on a box to fit these either under or behind the seat? All the boxes I have found are downfiring and do not have enough room.

I would suggest either getting in contact with supercrewsound.com, subbox.net, or proboxrocks.com and ask them if they can build a custom box for you. Or check out a local car audio shop and see what they have to say, but you will probably have to raise the back bench to get them in.

You will not be able to fit those subs or any 10'' sub under the rear seat without a seat lift kit. Contact one of the sites as mentioned above for a box- lift kit combo. If you do not want to lift the seat you can fit 2 8'' subs. Good luck!

Have you seen one with a seat lift kit, and if so, is there still enough head room in the rear?

I got my box from proboxrocks.com and it came with a 2" seat lift. I actually think it's more comfortable having the lift on the bench, but it is a little more difficult to fit back there head room wise, but then again I'm 6'3" and didn't fit very well before the lift. If you have kids, chicks, or normal sized people back there you shouldn't have a problem.

HAHAHA HEY!!!! i'm about 6'3" as well.
lol are you trying to say i'm not
normal?
as if to say i must be a freak?? lol jk

to answer the questio at hand now. NO you should not have any problems with AVERAGE size people.

See i prefer to think of myself as ABOVE average as opposed to NOT normal. haha