Floor Pan Questions

Just ripped out my nasty carpet and found lots of nasty stuff underneath. I have a bunch of question about what I'm seeing.

How many holes did the factory drill for seats in 1965? I have 8 smaller holes that I can tell are obviously not original. The larger inboard holes with the reinforcements next to the tunnel fit my bucket seats. Out board I have two sets of larger holes. The most out board holes were plugged with rubber plugs. These look like oringinal holes. Inboard from those are two more holes which fit my seats, but they don't look like the factory holes. I notice that the two sides are not symetrical with the passenger seat more inboard than the drivers seat. Also, the seat tracks are not symetrical. Given the 8 extra holes I'm thinking the stock bucket seats I have might not have come with the car.

The area of the floor under my front seats appears to be bent down in the middle. I am assuming that the reinforcing grooves that run left to right should be straight and unbent. Is this a correct assumption?

The front floor pans look solid, but the rears both have holes with the passengers side being the worst. Looks like the replacement panels for sale only go back as far as the seam. But I have rust holes behind the seam (see picture around the rear most drain hole). I'm guessing nobody makes this piece and that I will either have to fab something or find a donor car.

I'm interested in advice on what to do. Given the way the floor is all bent under the seats and all the holes that need to be welded shut, would it perhaps makes sense to get an entire floor from a donor car? Am I underestimating the work required to extract the entire floor pan from the cross member and all around the perimeter? What issues are encountered when replacing the hump? I assume the car needs to be well supported to keep things straight.

Advice welcome.