If they're too close/low down, it just encourages your spine to get crushed in an accident (sorry - bit graphical, but it scares me when I see them mounted that way!) As the direction of travel is straight down towards the mounts - so as you can probably work out ... thats gonna hurt.
Don't use the rear strut brace, it's not designed to take the horizontal forces in the direction the harness will pull it.
A harness bar is the best option, as it mounts the harness points at the best level - you need the shoulder straps as near to horizontal as possible, see MSA blue book :
http://www.msauk.org/uploadedfiles/msa_forms/bluebooks/09/Competitors_Safety.pdf
bottom of page p183 shows the kind of angles you should try and get.
You should be able to mount the eye bolts (3 of them) where the mounts were for the seat belts in the back of the car - use the centre one for both seats (1 left and 1 right).
To cross them over means mount the left harness strap on the right harness eye-bolt and vice-versa. I mention this as you'll probably not have the harness completely tight - for comfort on the road, so crossing the straps behind the seat stops the looser shoulder straps from splitting apart under load and allowing you to fly forward between them (try it out and you'll see what I mean )
Also, you should really only use the mounts on the car to secure any of the lap straps - not mount to the seat. It sounds like you have mounted to the car, so thats for the benefit of anyone else reading!
It's a little difficult to describe, so if its still not clear (my descriptions aren't always the best!) I'll take some pics of mine and show you were you can mount them...
After reading this, you'll probably see that theres not too many alternatives to mounting the harnesses elsewhere - best bet would be use a harness bar or the mounts that the rear belts used. (They are already reinforced due to the original use)
HTH's