Retractable ones you would have to drill a 4th hole in the side of the wheel well, and weld a captured nut there (at least i'd weld the nuts, so that you get the strength of the body along with it, and not just the tear strength of a bolt through sheet metal.
Personally I find the 3 point static harnesses far more comfortable than retractable ones in my tr6, because of the low mounting / pivot point for the retractor, it digs into my shoulders.