My '91 had a threaded nut, not just a hole, the '94 Supra seat front bolted up just fine.
If you know anyone who can weld, you could weld a tab onto a bolt and slide the bolt in from behind, similar to the bed's bolts, use a nut insert.
The no weld/new tool system, would be to take a piece of 3/8"x 3/4 or 1" flat stock, drill and tap a 3/8" for the seat mount and rivet the flat stock to the backside of the mounting area.
Once you get the fronts mounted, you should be able to get the rars, the thickness of the carpet is likely keeping you from getting the inner rear lined up.
