Frank,I have one of those moss kits and while I can give you definitive info on the seat padding, the seat back is not so definitive. It is a bunch of padding. Looking at my old seats it looks to be about 2.5" to 3" on the sides of the seat and 1.5" to 2" in the center. I haven't taken my seats apart yet so I can't be more helpful. I feel that someone will be able to help, though.
Maybe I should do mine just to get me out of the doldrums and doing something positive instead of "waiting for parts". In fact that sounds like a great project for next week!.
Good luck, Tinkerman