1967 Cuda FB project

The frame rail is welded to an inner cross brace. Then there's a space and then the outer rolled panel. I bolted the spit through the bumper mounting holes which are above the floor panel in the trunk. The frame rail is welded to the inner cross brace below the trunk floor.
Hope this helps.
And you probably have too much structure missing to put on a spit right now. My car has almost no rust so it is pretty strong for the spit. I took measurements before I did any spot weld removal and after frame rail was gone, no movement. I attribute this to the car being that solid.

You can see in your first pic the two holes under the trunk floor that look like bumper mounting holes. These holes are in the end of the frame rail. They are not bumper mounting holes. Maybe for a different a body model or previous years, I don't know. But, I do know the bumper mounts above the trunk floor through that roll pan you just put on there.