I was workin on a project for my friends 05 chevy extended cab and looked like most were downfiring, with a couple being forward firing, not too many choose the up firing design. Personally I would probably put them downfiring.
I just reread your message though, looks like you already have the enclosures built?? I forget the measurements but I think there was about 13" of room towards the front of the back seat but only about 8 or 9" towards the back. To downfire you would most likely need an inch or two between the sub and the floor, so it doesn't hit the floor when pounding, even even then I would worry about junk sliding under the sub and damagine it.
I guess without building a new box I would just slide em under the seat as far as they will go and up fire them. You could always build your own box and still have about the same internal volume when all done, just gets tricky when doing it all out of MDF seeing the floor isn't symetrical and the back hump eats up a lot of room too.