Tom:
I would go with 3" shoulder belts for the passenger side.....heck it's what I have on my driver's side anyway. I don't like the 2".
As for how to mount the seat.....mounting safely is important but luckily position isn't critical since the passenger isn't driving.
I recommend using some flat steel as a base.....and mount these to the stock mounting positions (those prethreaded holes on the front cross piece and the two rear humps. Throw a cross member in between. The two front to back flat steel pieces will be on a angle like a ski jump.
Then use side mount brackets to mount a seat to the flat steel base.
I would make that passenger side seat easy to get in and out......the reason being I would be taking it out to race (imagine crawling over it to get out if you had to!)
The kirkey aluminum seats are easier to get in and out than fiberglass seats, though instructors will want something similar to what you're sitting in (and I don't blame them). If you want to put a fiberglass seat in....look for one that's "expired". They are SCCA legal now w/o a back brace. May help you save a few bucks.
Good luck!
Vick
www.volko.com
Black SM/SM2/"Slap Bracelet Throwback" #12 in the Northeast....if the car was made in the early 90's it should look like it.
1.6L forever! Bring on your '99's and '01's!