Thanks for the warning staledale.
The ones I mentioned earlier say they have a steel tube frame: http://www.subesports.com/products/cat/Seats/brand/Cobra/prodID/1896
If I go with a Cobra replica type seat it will be reupholstered to match the interior so while I've got it apart I could weld/bolt-in a mechanism for a head rest...nothing is simple!
By the way, looks like a nice GTO Sidwell. Welcome to this thread...where we obsess about things like...seats, LOL. How about some more pics and more information.