Does your car has the steel strap that holds the front of the seat support down?

I have seen several 544s without them.

I would move the seat rails rearward and add some support to the rear of the seat. Perhaps a wooden block about 1 inch thick with a 3 inch wide steel piece to spread the weight out on the floor. An inverted T.

The support may not have to go past the rear of he seat bottom.

Your rear seat passengers may have to keep their feet between the rails.

I have 850 seats in my 544 mounted on an adaptor plate bolted to the original floor mounts.

I had just pulled the seats out of a 61 544 that I bought recently and then read your post that night. Like you, I am tried of either getting into a Yoga position or advanced gymnastic move to get in and out of the car. I added a couple of additional slots in a previous car but it just barely added a inch. So yesterday I moved both brackets back by just using the two rear bolts, and re-installed the seat. A lot of leg room created but it eliminated the same amount for the back seat passenger. I'm going back down to my garage right now and make sure the seat still latches in place and see if I could make some sort of additional support for the over hanging rails.