Welcome! Show us pictures of your seat.
I would feel pretty confident about using a '67 Seat. Seats are becoming so hard to find out there, that many guys are just repairing them. It will take some patience.I would not mind a speaker in the seat at all, as long as it fits. I could try to make the frame I have useable. It has some rusted broken pieces in some places. Maybe I will try to take some hog wire and weld it . Lol
I saw somewhere that any gm fastback rear seat will fit. (Buick, olds)you know, I'm thinking , does anyone know an old school auto interior guy who's been doing this stuff for decades? they might be the ones to know about the differences between the seats