All early Mustang front seats are removed in the same way. There are a total of four 1/2" nuts that need to be removed to free each seat (total of eight if you have bucket seats, four for a bench seat).
From underneath the car, there are access holes that allow you to get to these nuts. There are supposed to be rubber plugs that cover these holes to prevent water and debris from being able to enter the car, but they are often missing. Once you have located the access holes, you will need a deep well socket to remove the nuts. On some of the access holes, you can stick your finger in and get the socket on the nut fairly easily; on others, you have to get the socket on there by feel.
Once you have a nut loosened all the way, bring the socket STRAIGHT down slowly, as it is very easy for the nuts to fall out of the socket and get trapped between the seat base and the floorboard of the car. If this happens and you can't reach the nut by hand, try using a flexible magnet. The nuts are of an integral washer type, so use the same nuts or their equivalent when you reinstall them.
When you remove your seats, there should be metal shims that fit between the seat bracket and the carpet to protect your carpet from being torn by moving the seat fore & aft. If they're missing, replace them prior to reinstalling the seat.
submitted by Tennessee Cub at the Vintage Mustang Forum