[Peter Zimmermann]

I think there are two questions here:
What was original California equipment?
What are the current CARB requirements for an original CA car?

As corollary to the second question, are the requirements different for a 49-State version ‘imported’ into CA or a CA version, registered in another state and then reintroduced to CA?

1976 cars imported to CA must meet the same standards as an original CA '76.

Someone correct me, is MY1976 the first year where a CA registered car must meet all CARB requirements as originally delivered?

Yes.

If that is true, what are the rules on installing a later engine (’77 Turbo or otherwise)? Do other systems have to be updated?

The car must meet all smog requirements for the updated engine (in this case, smog pump, reactors, ex. gas recirc...), which were all fitted to a CA '77 Turbo Carrera.

I’m assuming the ’76 is a very nice car and worth considering.
Anything less and I understand you should consider the CARB exempt ’74 models. I understand there is some ‘gray area’ concern about ’75 being exempt.

'75 models are exempt in CA.

Here's a 3rd question. Why on earth would someone put a Turbo motor into a narrow body car? The boost and over steer were vicious even with the big flares, and wider track/tires. In a narrow-bodied car it's got to be borderline undriveable. 235 hp is cool, but not when it occurs all at one time like an on/off switch!