I would say you have the correct back for the bottom. apparantly ( and please correct me if I'm wrong) but the seats for these trucks came as spring back/foam bottom, spring back/spring bottom and foam back/foam bottom. now, I dont know what years had which combo, or if it was a trim level difference.
it looks like lmc has you covered on the foam bottom, but they dont do anything for the spring units:
http://www.lmctruck.com/icatalog/cb/full.aspx?Page=47
not sure if that link will work
and as for swapping backs..hopefully someone can chime in on that :P