One thing I would do is leave the large bolt at the top for last (the one on the B pillar). That way the belt will stay out of the way while you're trying to get at those 4 small bolts at the bottom.

It also helped a lot to have the rear seat out of the way so I had room to work. The bottom part of the rear seat is the hardest to remove. With one foot on the rear floor, put your knee into the bottom edge of the rear seat, just above the carpet. You have to push in (towards the rear of the car) and unhook a metal rod in the seat from a metal hook on the floor. So, you push in, and then pull up. I had a really hard time pushing in far enough to unhook it, but once you get one side, the other side is easy. Then there's another similar hook in the middle right in front of the armrest. Pivot the bottom of the seat up and push in to unhook it. The rear is easy. On either side there are two visible metal tabs that you bend out. Once those tabs are bent out and the hooks are slid out, just push up on one side of the seat back and it should slide up and off of the hook. Same for the other side, and then you can remove it. With the extent that the carpet needs to be pulled back, this should make things easier. I know it did when I swapped the front seatbelts.

Might as well pull up the carpet as much as the seats will allow and check the floor for rust. I found a couple of minor spots in my '87 240, which has NO rust on the outside, so if your car looks clean, you might not be totally rust-free.

Good luck.