The NMCA N/SS rules have been pretty much the same for years. No, Camaros, Mustangs, , small blocks, 68 & up full size, etc. It is what it is. Just like any other class. If you want to run in it, build a car that fits. That being said there are plenty of Stock, S/S, N/SS races that will accept a "close" bracket car. There is no engine tear down. But it just like N/SS you need to have a correct appearing factory carb. No Holley DP or Dominators, Factory style interior, no tin.
Doug