There are some photos at
https://cj3b.info/Seat.html which give an idea of the early and late seats that Heinz is talking about. Those photos suggest that there wouldn't be a big problem installing early seats, although I guess my question would be -- why? I assume because the existing seats are in poor condition, and you have a pair of early seats you can use. But still, would it not be less trouble to get the existing seats repaired? Just curious.
Derek