Welcome to lilhonda Jane.
CT70 seats are notorious for getting cracks in the seat pans, and they're tricky to weld up. The original foam tends to deteriorate after 40 years or so too. You are planning to shave the foam down thin, so you probably want to start with some really high quality foam so it's as firm/soft as possible when only a inch or 2 thick. Then you will likely be making a custom cover from scratch.
So, I'm thinking you might want to find your upholsterer first. Maybe take your original seat with you so they can know just what the job will entail. Then let them advise you on how to proceed. Maybe they will want ONLY a seat pan, and they will do the rest. Maybe they will want the foam too. If they want the foam too, you might want to buy a good quality brand new reproduction seat...with quality foam. If you're gonna make the effort and investment, you should do your best to get it right the first time.
I'm thinking the upholsterer will know what foam is best for using in minimal thicknesses. And they'll know where to find it. We can help you find the seat pan.