Those guys were highly protective of two things: rear suspension design and cylinder head/intake design. And none of those details made it into the kits! The early ('71-'72) Jo-Han and MPC pro stock kits were okay because the suspension layout remained close to stock. Once the fabricated rear half cars came into being, the kits lacked the proper detail. It wasn't until the 1983 Monogram kits that the accuracy came back. Everything in between was basically shelf material/don't look too close when it's turned over...
--Mark