This is just personal and my opinion only... but the one thing that turns me off every time when I'm looking at a vintage car is when someone has replaced the original seats with ones from another, usually later, model. Unless it's a street rod I want to see the original-style interior. I guess that comes from seeing too many junk cars where someone did a quick fix by replacing a trashed set of seats with whatever fit that they could get their hands on.
There is a really cool '65 Fury on ebay right now for big bucks where the owner ripped out what looked like a great original interior and replaced it with "custom" gray mouse-fur velour because, he said, the original was giving too many static shocks. For some reason he posted a few pics of the original interior that is no longer there along with ones of the one he put in. There is no comparison - the interior that is in there now reduces my perceived value of this car by thousands, although it is extremely desirable otherwise. The same holds true for a really pretty '70 Monaco that is for sale over at the Drydock - the owner did a reupholster job on the seats that just detracts from an otherwise beautiful car. I would rather have worn original seats than seats that do not belong to the car if we are talking a collectible car. If we are talking a beater, then I concede that all bets are off.