Hey Michael,
Couldn't help but to respond to your post. Even though I don't have any other experience than using my stock seats and upgrading to a 4-point harness set-up. I have a 69 GTV with the stock "flying buttress" seats with a Sparco 4-point harness, and it works nicely for me.
The trick in this particular set-up is a proper harness bar (complete with loops for the harness). I had this built because not only did I not want to replace the stock seats, but I also don't want to modify in any way the interior of the car - which includes welding or drilling of holes for a roll bar.
At the same time, I really want to have good, safe fun at track/lapping events (no real racing yet). This bar bolts into the existing shoulder strap holes and the rear lapbelt holes (at bottom of wheel well). Best of all, it can also use the stock Klippan bolts!
If you're interested in this type of set-up, let me know (PM me) and I could send a picture. I've also got bars for other models in the works.
Just listed it on eBay:
http://cgi.ebay.com/ebaymotors/ws/eB...tem=2466149708
Mirko