I took the seats from my '70 to an upholstery shop
They said there was nothing they could do with the springs. What they did was take very dense foam "sticks" and really twisted them and they placed these within the spring area. When I went to pick them up, I sat in the seats and told them "ok, it's alot better. Now can you make them twice as firm as this?" They took them apart again and when I picked them up the next time, they were great. They have held up very well for 3 years now, and they are comfortable. The shop was in San Diego. If you are near there, I will reply with the name of it.