[Sheepdip]

Smog pumps or A.I.R's were first required in Kalifornia in 1967 and they ran till about 1975 or so when the Catalytic Converters were introduced.
Different trucks/cars had different rules e.g. C10, C20, C30 also different engine sizes entered into the equation also so it's a twisted tale to get it all straight in your head for sure if that's even possible.

Rumor has it too (and I could be 180 out) that the 402's we find in our trucks from the factory was a GM smog game, seems 400 Cu. In. and below was the cutoff for anything smog related such as compression etc. The 402 Cu. In. put them above that threshold.