Legendary interiors install question

I am also having trouble with my legendary seat covers on a 67 Barracuda. The foam was not good enough to reuse so I ordered new foam. The FSB foam and covers worked just fine. However when I attempted to do the FST (front seat tops) the foam made the cover bulge and look very pregent. The foam instructions said it may need modifications, but it just seems to thick! I cut the channels in ok, but the piItes in the seat back are a little too loose with the original foam and too tight with the new foam. I already cut the the channels in the foam am wondering if the foam needs to be shaved or something. It also said not to glue the seat cover to the foam so I can not use the original seat back foam. ( too loose) I what to use the new foam as to the fact that I bought it already. I have not made an attempt at the one remaining seat back. Can any one please advise. Thanks Caxey1