Just today my son and I replaced the seat foam cushions as well as the covers (new leather to replace old and torn vinyl, and new foam inserts) in his '73 Datsun 240Z. The funny thing was, after we finished, we found we had to take out the 1 inch spacers underneath the seat runners, because our head was touching the roof lining now whereas before there were at least two inches free room! So the new foam cushions added about 3 inches to the 'sitting height' and the comfort difference is really night and day. Agree with Achim that renewing the foam/springs will make the difference in seating comfort.
When I bought my Pagoda, the seat covers were the original ones. The strange thing was, after a few years, the cover of the passenger seat started spontaneously tearing, even though that seat was used much less than the driver's seat. After a few years the tears were so bad I had to replace the covers (and Shvegel very kindly sent me his used ones in good condition, same color).