If whoever built the car stayed with the stock seat hardware then the bottom cushion drops into a pan that is part of the floor structure, so not exactly as some might think from the description. It should have some locating lugs. If there's concern about it "flopping" about it can be located with a couple of those spring lugs shaped sort of like an arrowhead.

Each state has it's own view on the seat belt thing, they weren't federally mandated until '67, though as Pops mentioned they were an option from some companies prior. That being said, it's silly to not have them (properly installed).

One more thing you might consider are door latches if the car has only the stock ones. Depending on how the car was built, Model As can be pretty flexible. It's always.......uh.....interesting.....when you power through a turn and your door(s) swing open. Might scare some little ol' lady on the curb too.