Just an update: Number Dummy you are a wealth of info here....... but......This time hands on rules the roost.
If you are going to find an USED set from a 53-55, they will not fit because they do not have the modification for the gas tank. IF 53-55's were the same as a 56 the part number wouldnt have changed to show the update in 56 with a 56 part#
Now the 56 part # will fit in a 53-55 as there never was a gas tank in cab and 53-55's dont care about the update.
I have the correct seat tracks from a 56 if you are interested.