Are you sure it's your seat springs, and not worn out seat foam that is causing you to sink in your seat?
The 70 and 71 seat foam are reproduced, but not the 72-74. You will have to find a local upholster shop that might be able to help you there.
The spring issue I can't help you with either. I mean, if it's as bad as you say, maybe you can find a seat on E-Bay or Craigslist that looks like garbage, but has decent springs to use as a donor. The seat cover can be swapped if it's in good condition. You just need pliers, hog rings, and hog ring pliers.