bucket seats in a bench seat car.

So the giant square washers I used won't work? :shrug:
What gauge steel do you suggest?
Whatever works for you works for me. Heres how the factory did it

I've been into seats restoring and found the frame cracked allowing the outer corner to sag down. In that case the crack is welded and the seats are switched left and right. First corner set on is renewed.
The foot print of the seat track could cave the floor pan but on the inboard side of the seat that is very unlikely. The plate does double the size of the footprint and floor thickness.