Wes, the person you bought your harness off is also selling a roll-over bar which has harness mountings on it I think.
I have a roll-cage so that's where I mount mine (on a specially provided 'harness bar').
I would think if you were to have a very strong bar that either bolted to the rear strut top mounting or was welded to the towers that would be strong enough and at the correct angle.
I have seen them bolted to the rear deck too, but it would need re-inforcing probably with a plate underneath. Have a look at the Turbo for sale on this site.
The straps need to be at about 45' . If they go straight back horizontally or nearly vertical to the floor it wont hold you in your seat - you could move up and down or forward and backward.