Here are a couple of thoughts on seats:
1) Safety really comes first, and may be a concern to others, too. (It is to me, having been rear-ended in a go cart race this winter, which resulted in many trips to the chiropractor and on-going neck exercises. BTW I was only hit at 15-20 mph: consider what it might have been like at higher speeds) With that in mind any seat with a removable headrest offers the best of both worlds, if preserving the original appearance of your car is of value to you. (this option was covered by an earlier poster, who mentioned Subaru seats from the 90s)
2) Your comfort is, of course a close second. This may not be a concern for you, but I am 6’7" and a difficult fit in some car/seat combinations. (you might ask why I am a Sunbeam fan, but that’s another story…) I find the Series III-V Alpine seats very comfortable.
3) If you want to preserve the appearance of your car for shows and / or special occasions, then keeping your original seats and finding modern seats that offer comfort, safety and affordability is an option. To that end, I am buying, but have not yet seen, a V6 Alpine with Mazda Miata seats. These have a built in headrest (with speakers) and fit comfortably into the Alpine. Whether they fit the existing seat rails I can’t say, but they look good in the car, imparting both greater safety, more recent foam (not the well-known dust blocks the hardened, brittle Sunbeam seats have become with age) and a look that works with the car’s roll bar, minilites and other updating features.
4) The cost of restoring the seats is considerable, if they are done well. Buying an interim seat for safe comfort could also buy you time to consider the new seats’ virtues. If you decide to keep ’em, you can sell off your Sunbeam seats and preserve your options in the meantime, amortizing the cost of the new seats with the sale of the old.
Hope this is of help. If you are interested in the Mazda seat fitment, email me at cdk84 at hotmail dot com and I will let you know how they fit, perhaps with pictures, when the car arrives.