Besides the headrests, in my mind one reason to go with aftermarket or later seats is that there have been numerous upgrades to car seat safety since the 60's; headrests are only one of them. I have put a lot of time into trying to find the perfect seat for my SIV V6. Perfect for me includes a separate headrest, as I just think it looks better and I like the idea of just pulling them off and still being able to use a tonneau. After several false starts, I think the Mazda M Edition seats will fill the bill. I don't have a photo of them test mounted in my SV, but you can see what they look like here:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&vxp=mtr&item=380378883652 I believe these were only offered one year in the US, so they are hard to find, but they do come up. I bought mine on eBay a few months ago for about $250. They will need to be recovered and I haven't decided yet if I will go with these seat covers or have them custom made. I thought they looked very good when I had them in the Alpine and they were more comfortable than the stock ones. As Rich noted, I did have to mount them as close as possible to the floor to keep my thighs from resting on the stock steering wheel, but it can be done with some trial and error and modification of the stock Mazda seat rails.
Just to save someone else from the same blind alleys I explored, my first attempt was a Procar Rally seat.
http://www.procarbyscat.com/Products_Rally.asp I really liked this seat and wanted it to work, but it was too large and sat too high for my tastes. However, it might be a good choice if you don't mind that. I have a photo somewhere of one in my SV if anyone wants to see it.
My second attempt was a set of1977 Fiat 124 Spyder seats that I was given. The seats came off a parts car that had sat outside for a while, so they are really beat, but I thought they looked really good in the car. Here is a picture of them sitting in my S3:
I liked the look, but they also too sat too high for my tastes.
The Mazda seats were my third and most likely final attempt and I was able to get them close enough to the floor to be virtually identical to the stock seats. However, for those wanting a headrest with stock seats, there is another option that I went with on my SV. I had a roll bar made for the car about 5 years ago and designed it specifically to allow a headrest to be added. It took me until just recently to have them made, but here are two shots showing them:
The headrests are made out of plywood and foam and held on with muffler clamps. I paid $70 to have covers made for them and am very happy with the result.