I'd mentioned what the rear speakers were mounted on in my write-up, and there's pics on the forum here as well. I thought you'd already known. lol!
I don't think the seat-belt mechanisms front/rear are the same, but I could be wrong. If it's like most other cars though, you can probably fix the fronts being "slack" by "torquing" the spring in them 1-2 more rotations.
I submit that a speaker box in place of the stock rear speaker location isn't really going to net you any advantage, unless you're trying to make them sealed enclosures. Instead, I'd recommend doing what I and others have done, and dremel out the mount to fit bigger speakers. If you want specific details, just ask. I'm happy to help.