Rebuilding convertible rear seat bottom frame

I have similar plan in mind. My vert seat bottom is not rusted beyond repair. Worst part is sides and front curves where perimeter frame meets the floor. Haven't attempted yet or decided would it be easier to turn the rust free notchback seat bottom into a vert seat bottom.
I noticed the springs in my vert seat bottom are not level. 2nd spring from each end lower than the others? I forget exactly. Something looked odd.
There was a nice vert seat for sale here but... in Canada. Could have been offered complete back and bottom too (I forget).