Hey dodgezilla, I had the same problem with my 73 challenger seats, they were the injected type where the bottom seat bun could be replaced but the back portion was literally formed onto the seat frame and nobody makes a replacement type. After doing what I could research wise I found a good video on Youtube that legendary auto interiors did on a 1973 challenger/cuda and decided to tackle it myself. now I have no experience doing upholstery I grew up on a farm.
what they did was add in 1/4 inch foam on the bottom bun on the middle section and both side bun sections. then they did the same thing adding in a 1/4 section for the upper portion middle area, then on big portion for the surrounding top and side areas. So I tackled this myself with my seats and I was quite impressed on how they came out, the hog ringing is very easy but I recommend getting 300 or so if your going to do it as I ran out and couldn't find the right ones at any part store (home depot, lowes, tractor supply, big r(Colorado feed/tack store)
I will post a link of the video and pictures of how I did on my front and rear seats on my challenger project for you below.
https://www.youtube.com/watch?v=-KbXNg_HUCI