You're in California -you are lucky to be able to fit an aftermarket air filter without arousing the law.
I'd give up on engine swaps outside of the 6Vxx family. Even the DI engine will be a CARB challenge.

I heard that Mercedes-Benz invented a 100 mpg carbureator during the 70s, but the entire engineering team was kidnapped while vacationing in the Middle East.
Any thoughts Joe?