The issue with these old cars is there's nothing to brace your legs on. Then I'm hanging on the steering wheel trying to stay up in the seat.

its kinda gimmicky, but i know guys that use this with OK results. not safety, but for autox stock classes where you cant change your seat, just to keep you in your seat:
http://www.jegs.com/i/G-Force/471/4290BU...CFdgNgQodeiYA3A

a three point is not going to work without a passthrough/adj headrest posts or something to locate the shoulder straps. Then you need a harness bar so you don't compress your spine (and anchor to) if you actually hit something. Otherwise i would only consider a three point for autox pylons.

one of you enterprising types need to make a passthrough bezel in the manner the SRT4 ACR seat bezels that are no longer available. EDIT apparently my google foo is weak and ive been looking for the wrong thing. search "seat grommets". like such http://www.c5racer.com/catalog/c5-c6-z06...ness-belts.html shouldnt be terrible to install again you need belt higher than your shoulders.