Spare tire well removal

The flat floor is relatively easy to do, and it does make a decent amount of room for a larger tank.

As for OE tanks, you're better off going custom. The E body tanks don't have an option with the filler neck on the left hand side. Challenger tanks use a right side filler neck, Barracudas and the '71 up B's used a rear fill behind the license plate that won't work with the A-body rear bumper and structure unless you put the fill inside the trunk, which causes other issues.

I installed a 68-70 B body tank in my Duster, but, it takes a ton of work. The filler neck is close enough to use the factory A-body hole in the trunk pan, but the tank is too long. I kicked out the rear crossmember almost all the way out to the bumper to get the tank to clear in the back, and had to move the triangular supports. The process would be a little different for a Dart I believe because of the differences in the rear bumper. It does make a bunch more room for the exhaust, and the tank doesn't hang down as low as an A-body tank.