is it safe to use an original rear belt fixing point? I know its only one of the 3 points used. Im guessing not really
No. You're going down at approx, or over 45 deg. In a crash the harness will try and compress your spine. Hideous, nasty, dumbf*ck idea used in WAAAAY too many cars as a shortcut. You should have a harness bar in the cage that you need to install to get the 0-10 degrees angle which is best practice (see the MSA Blue Book for full details).
where would be the best fixing points for stress plates and eye bolts for harnesses?
Mounting plates should be welded to the shell underneath, so the nut is on the underside. See the Blue Book for detailed recommendations concerning this stuff.
Rummage at:
http://www.msauk.org/ for further details instead of relying on other people's half-assed nolidge.
B*ggering about with your seat and belts is a stupid thing to do in a half-a$$ed manner. Do not enter into it lightly.
