Hey Fred,
I am in the process of changing my seats at this time and the best way to do it
is to change them already mounted on the foam. They just snap in with a couple of
wires in the back. Your 1981 should have the 4'' Bolsters from the factory.
I was unsure of the 2''-4'' thing but I bought a pair of clamshell frames from
salvage yard and it just happened that on has the 2'' and the other has 4''
but since I am changing them does not matter
Good Luck with your install...good info on Corvette America website
Happy Motoring
Andy