M715 Spring Seat
I know this is old and you probably already replaced the seats, but.
Take the canvas off, take a GI foam sleeping mat, cut it in small strips with a width the height of your seat springs, roll them up and put them inside of the spring coils. What you have done is made an inner spring mattress. Then cut a one inch foam pad and place on top of the springs, if you use some more sleeping mat, it is about a half inch, run it down over the front, reinstall your seat cover and you will have a very pleasant ride. Advantage of using GI sleeping mat is that it is impervious to water.
You can vary the height of the foam in the coils to get a stiffer or softer ride. I am about 200 pounds and have the foam to the top of the coils and my spine is no longer the shock absorber for the truck. I worked in an upholstery shop when I was a kid and the old man told me the trick to seats was to add an inch of foam, no one sees it and it makes a great ride difference.
Hope this helps.