Since they don’t have the T's they don’t need any extra reinforcement in the body. The hardtop keeps everything in spec with minimal body flex. They also skimp a few things found on most Z's, only manual tranny, no Bose, no power seats, no HICAS. While this saves a little weight its nothing a base model NA couldn't do. The big savings comes in the body. The slicktops where sold as the Econo 300ZX, but were not very popular. I think Nissan only made enough of them to use the body for racing, but I don’t have any facts to confirm that. It's just a theory of mine.