Ok, I\'ve done this:
First off, get a harness bar from http://www.rotaryextreme.com/
I paid $325 last August.
This acts as a rear strut brace, but is beefed up and has 4 eyebolt holes for driver and passenger side harnesses.
Then buy a 4 or 5 point harness from demon tweeks. These come with the eyebolts usually, but make sure.
Make sure your seat has the holes for the shoulder straps
The shoulder straps are at a near perfect horizontal angle doing it this way.
Mount the lower harness straps on the existing seat belt mount points.
If you\'ve gone kinky and bought the 5 point ball cruncher strap, you\'ll need to drill a hole in the floor under the seat, and adequately plate to ensure the strength is there. Make sure (again!!) that your seat has a hole for the 5th strap.
I have retained the existing belts as well, to keep it road legal.