I did extensive research on seat upholstery kits years ago. Naturally there could be changes in production in the marketplace since then.
What I learned was that Seatz manufactures two different style 240Z seats based on production year. The primary difference is the length of the seatback material, since early years did not have recliner mechanism, and later years did. Distinctive industries only manufactured one length of seatback with the thought that the extra material could be tucked up into the seat back.
Distinctive Industries manufactured the foam kit overseas to meet the specifications of their seat upholstery kits. Seatz does not manufacture foam kits and relies on dealers to offer foam kits to those customers who need them.
Since each kit is hand produced (both Seatz and Distinctive Industries) there inherently will be variation in fitment to either original seat foam or aftermarket foam kits.
To make matters more complicated, some cars have had their original seats replaced over the years, resulting in non-year specific seats installed in a dated car.
With all these variables of mind, it is not unusual to have to shape the seat foam by either cutting the foam or adding additional foam.
I personally went through three sets of seat upholstery kits, and ultimately had to return a set to MSA in order to have Seatz rework a seat back.
Persistence is the key to getting the seats to original. This project can easily exceed 10 man-hours.
Keith