Tonight, while offering advice not he FAQ regarding my experiences using foam for the rear, I decided to break out that set and go for it. I tore my seats back out (3 minutes or less) and pulled off all the covers. It went real dang easy. And the final product isn't too shabby! My covers were stretched a touch from the puffier stuffing, but it wasn't bad.
The 1973 cover..
My cover on the 1973 frame!
The back 1973 stuffing exposed.
Then add some 1975 love..
After the above picture, I went back of the set and re stretched to be sure it was as good as it could get and here is the final product.
So, it appears that it's good logic to score a set of early seats and reuse the entire frame, with gummihar stuffing, and wrap them with the later '74-'76 vinyl.
Cheers!