Floorpans - Best Replacement Option?

Replacing the interior floor with a one piece AMD floor is the ideal way to do it. Much less welding than doing two half floors. Two half floors will require nearly 10 feet total of either butt welding, or lap welding to the trans tunnel. Lots of fitting, and a visible repair from below.
By taking out the whole floor, you now have full access to the inside of the torsion bar crossmember. This is a known problem area on A bodies, and being able to access, clean out, and make any necessary repairs is a major bonus to replacing the whole floor.
There is a boxed in area at the rear of the floor that overlaps the underseat pan. This is also a rust prone area. With the whole floor out, you have full access to take care of that area too.
Seeing that your car has a big smashed area on the right rear tunnel, that would be taken care of too.
But either way, it isn't a job for a novice. Let someone with experience handle it. There is a reason why auto body repair is a minimum 3 year apprenticeship before doing serious repair like unibody structural panel replacement.