If it were an S13, taking out spare and rear seats would lose you about 40 - 55lb, I'd guess. Spare is about 25-30lb, and seats are 20-25lb.
Rears STB would help some, but as far as I'm concerned, STBs don't do as much as people say they do.
Also, keeping the spare tire in there will make it easier to swing the rear out; I now keep a full sized spare in there for just that purpose.
