Drilling floor/rockers for seat belt mounts?

A nut welded to the sheet metal isn't strong enough. It would rip right off in a accident. If you looked inside the rocker you would see a larger, thicker, steel plate welded where the retracter bolts on.
Those belts have been installed in a-bodies having bucket or bench seats. i dont know if anyone has found a good method yet.
You may find the factory anchor point is too far forward and the aftermarket retrater hits a bench seat. So instead of one more hole forward of the original you actually need two new anchor points, one forward and one further back of the original.
Good luck