For a firmer seat, use a piece of burlap on the new seat springs, followed by a piece of thin composition carpet padding (the one made of scraps that is very firm), a layer of batting, a layer of cheesecloth, and finally the seat cover. This will give the feel of a modern seat without affecting the styling. You can pad out the bolsters on the sides of the seat covers with cotton batting (looks like felt) so that they have some definition, and be sure to wire down the listing wires to the seat spring frame for proper appearance.
Vikki 1969 Goldenrod Yellow / black 400 convertible numbers matching