Regarding the studs that go through the floor, a few of mine were badly rusted and I was able to press them out of the bases using my bench vice with a socket on the head side. It slightly mangled the base but it went back in shape fairly easily. Some heat added wouldn’t be a bad idea and use nuts to protect the threads if you’re reusing them. I replaced the studs with some grade 8 carriage bolts that I welded in.
I was discussing this with a restorer the other day. Mine were in the middle holes on each end and he had a set that used the front holes. We speculated that the factory probably used the middle holes and that the selling dealer might move them if requested by a shorter or taller driver.
The configuration of my seat bases (‘67 Barracuda) are different than the first picture in post#6. I’m not sure what the OP’s seat bases look like.