If you want to use those exiting snaps your best bet is to have a local upholstery shop make one up. Use UV protected vinyl, 18-20 oz.. The odds of finding a ready made that has the same snap spacing is just about nil unless you know who made that one and they still use the same pattern.

Most of the commercially available ones today are made to snap to an aluminum frame work that rests on top of the bed rails, the snaps move in slots in the frame so they are infinitely adjustable. If you wanted to go to one of those you'd likely have to find a more resent application that's close in dimensions to what you have as I doubt anyone makes "mass produced" ones for that old of a truck.