quoted from a "friend" on another site....
It was once explained to me by a zone manager for the EPA.
MY UNDERSTANDING IS:
(A) Before the 1968 model year there is no restrictions.
(B) An older engine may not be installed in a LICENSED vehicle newer than 1968.
(C) A newer engine may be installed in an vehicle 1968 or newer, but for smog emission purposes, the vehicle then would be the year of the engine.
Examples of (B).
It would be unlawful to install a 1970 engine in a 1974 LICENSED vehicle. It would be lawful to install a 1970 engine in a 1974 non-licensed vehicle (race car, farm truck, etc.)
Example of (C):
It would be lawful to install a 1996 engine in a 1970 vehicle; but for emissions testing, the 1970 vehicle would have to pass 1996 emissions.
As stated earlier, THIS IS MY UNDERSTANDING, BASED ON A CONVERSATION WITH AN EPA ZONE MANAGER, and is the philosophy we use in selling/modifying late model (1968 or newer) carburetors.
If you plan to do one of the above, WRITE to the EPA and get written authority for your specific request.