Insurance and swaps.

Californian here. As long as your car is '74 or older, there's no problem. If I recall correctly, we used to have the emissions exemption roll over each year so that if your car was 30 years old, it was exempt. however, they stopped the roll-over in 2004.

That's correct. It's the main reason I sold my '83 Hurst Olds to help fund the build of my '69 Dart - I didn't want to hassle with trying to get the non-stock Olds 403 engine in the Hurst Olds through registration and smog certification.
I can do anything I want with my '69 Dart.