Here's a REPLY TO ALL.
1) Via Manci--I'd never use SS bolts here. SS is way too weak, and I have extensive experience in using SS bolts and hardware from my ownership of a car wash. While I would not use SS, if I needed to replace these, I'd go with what are called "small pattern" Grade 8 bolts.
2) Naj--I think I have the correct bolts, as all 4 of them had a 10MM head. That's the only set of tools I had. What I did use to get them out was an offset 10MM box wrench. Slow going. VERY slow going. But, the existing bolts are in good condition. I did not determine or even think of looking if the threads were different front or back. I placed each bolt back in its spot for the winter so I don't lose them.
3) Garry, I have heard of the pool noodle work, and thanks for the photos. I would have thought, however, that an additional length of noodle would be appropriate where the springs curve around at the front of the seat, to either dampen or minimize the compression here right at the front/leading edge of the seats.
4) Another thing I thought about was either replacing or doubling up the springs to stiffen the seat a bit more. Anybody try that?
5) The guy who restored my car was a good body man and a good painter as those of you who have seen my car can agree. What you didn't see was how good he was on interior things like seats. So, I suspect he had the seats completely apart and put them back together with any needed repairs, "to stock". As rmmchl indicates, stock isn't necessarily comfortable, and I add that this was 10 years ago.
I will have the comfort and leisure of time to do this, this winter.
Anybody else, let's hear it!