1967 Barracuda conv rear seat

So now we have 3 different situations here. My vert rear seat parts aren't rusted nearly as bad as yours. Another member has no bones to start with.
I'm going to share my morning coffee thoughts for your consideration... If I go out and take the rear seat bottom out of my 96 camry I wont find any springs. There is a simple wire frame surrounded with hard foam. These newer car seats aren't built like mattresses but a childs car seat doesn't sink down into them either.
You would be hard pressed to efficiently media blast your spring frame. Media goes right through it. So dip it in rust remover, half at a time if necessary (depending on size and shape of tank, a kiddie pool and vinegar?).
Once clean and dry, coat it with paint or something. I've painted intricate wrought iron a couple times. Sponge or rag with a rubber glove works a lot better than a brush.
Next, go to Staples and pay about 30 bucks for a large roll of cling wrap. Its about 24 inches wide. Wrap your frame very well. Cut a few holes in it as needed so to spray fill it with expanding foam (whichever density you choose, lower density expands farther and allows more spring movement).
You will still need foam or batting to fill out the reproduction seat skin but your foundation is no longer a situation.