There are a few manufacturers of the seat foam and only one that is really any good. New foam will definitely be thicker due to the many years of the old foam being compressed from the seat covers and someone sitting on the seat. You may have the wrong seat foam application for your car as it is different for each year and style of seat cover. I assume you are working on a 67 or 8 since you do not have the seat springs molded into your original foam, unless it is a very early 69. I have a few sets of seat foam here for upcoming restorations, and none of them are as thick as yours. How about some more information as to where you bought the foam, your year of car, and whether it is deluxe or standard interior?......................RatPack..........................