First question: are the seat runners the same bolt pattern for the Spider and Coupe? Makes it easier for me to hunt and look at other solutions people have tried.
Second question: any suggestions for an alternative seat which might look reasonably 70s (lowish seatback, not too bulky) yet also offer a reasonably adjustable headrest? Fabricating a bracket to adapt is not really a problem for me, and I would also consider reupholstering all seats to match. Some ideas I had were the first generation VW GTI seats (these were fabulously supportive and yet quite small), but maybe that's still too modern. I also thought the BMW E21 or E30 seats were fairly simple in appearance. My left-field idea was (don't laugh) a Lada Niva. I saw one in a classic magazine, and the seats look very 70s and Fiat-ish.

Open to suggestions.
The static 3-pt belts will also be replaced with intertial belts, but this seems quite straightforward.