[MikeP]

The usual method is to make a bar which bolts in inside the cabin across the top of the seat backs. The bar is good and solid and bolts through into the strongest points at the ends and also through to the angle section which goes across under the window. The threaded inserts for the belt mountings are welded into this strong bar. The bars can be obtained from Wealden Engineering, Southern GT, MDA etc or you can make up your own. I have attached a pic of a typical example. Hope this helps.
Cheers
Mike