Davids suggestion about using a roll over bar is a good one if you wish to go down that route. If so it would be worth looking at the seat belt arrangement in an SVA approved TC. This puts the inertial reel in the centre and stalk on the outside. Sounds a little odd in conventional terms, but actually works very well, is easy to use and most importantly, comfortable.

On the standard set up the centre stalks usually have a flat strip hoop under the tunnel area with two captive nuts on them that the seat belt stalks bolt through to. Alternatively, you could upgrade the chassis with the SVA cross members. You should also look at fitting the shoulder bracket to the seats if they are NG items. A little extra gusseting is required, but they have prooved to be strong enough to pass pull tests. Again you really need to study some SVA approved TF/ D's