Im working on almost the exact same problems with the same materials! although mine are unfortunately a little more widespread due to some botched body work initially done to repair it.

For some temporary stuff though until you get to it, id weld a piece of angle iron in under the seat belt bolt points on both sides. Mine is rusted the same way, and that sucker flexes like crazy when you lean forward. It would probably not work too well keeping you from going into the windshield in an accident.

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