My CJ-5 still has the original seat springs. The driver side is getting a little soft as you might expect.
Using the ebay springs for replacement parts is a good idea.
Do you have a source for the small tension springs that are arranged in an "X" between the coils? I found that tightening those helped firm up the seat a bit, but they are tired too....
I only sat in a 3b once, so don't recall if it is more like the 3a or the cj-5 in terms of leg room and seat height. I wouldn't mind sitting a little lower in the 3a but the leg room would suffer a bit....the 6" foam pads are plenty firm, I've been waiting 4 or 5 years for them to soften up a bit but they are not giving way at all...