I just fit a nice pair of 8's in my neighbors ranger, and they didn't hinder the adjustment of the seats all the way back. Might think about it, 8's are cheap and will provide more low end than the 6z9's.
Depends, I would go to a local audio shop and see what they have to offer for your ranger. I know they are not the best, but have you thought of using SAS Bazooka's? Check them out on crutchfield, and you could strap them down under the rear seats, or see if the audio shop has a prefab box that will bolt in under the rear seats.
Well, From my experience.. 6X9 boxes are the last resort..You get no bass out of them. I had my kappas in 6x9 boxes..and then I built a big 6x9 box..I bet I got 75% more bass,and its better bass..Its not that poping bass you get from 6x9 boxes.. The Kappa 693.5i give out some thump,but with ear piercing high's..sound even better on a nice amp.