If you get really keen, you could push a bolt through from the outside, weld it in place at the right height, and bobs ya uncle you have lap/sah belts at the right height. Most inspectors wouldnt have a clue about where the belts are supposed to be on 1000s so as long as it looks tidy they wont bat an eyelid.

You could also do that from the inside with some tricky welding. Depends on how keen you are.