Can you replace an alternator with a reg inside in 1970 Pontiac gto?

No, you can't directly replace a 1970 Pontiac GTO's alternator with just a voltage regulator. They are separate components. The alternator generates the electricity, and the voltage regulator controls the alternator's output to maintain a stable voltage. They are integrated in later model alternators, but separate in a 1970 GTO.

Replacing the alternator involves removing the old alternator and installing a new one (or a rebuilt one). If the problem is with the voltage regulation, you would replace the *separate* voltage regulator, which is usually mounted on the fender well or nearby. You wouldn't replace the entire alternator for a bad voltage regulator.