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If you are talking about the cage nuts for body mount #2 and #3, just center them over the hole in the frame and weld both sides of the cage to the frame.
The number 4 nut cages are riveted to the number 4 mounting bracket with the fiberglass body panel sandwiched in between. The rivets are aluminum. If I were doing a body off resto, I'd think about using steel rivets or stainless steel screws instead of the rivets. The aluminum rivets will shear easy, making it difficult to remove the number 4 nut.
