1. A couple answers, loose or worn belt, or the regulator is set too low.
2. Battery would never be fully charged.
One thing you might do, check to make sure there is ground straps between the engine and body, and body to neg terminal. Just provide a good ground for the alternator.
There is a buss bar on the horn relay, right next to the regulator. Remove neg cable from the battery, remove the two red wires from the buss bar and clean, wire brush, sandpaper, the two terminals and the buss bar itself. The same goes for the terminal block on the radiator support in front of the battery. There will be two red wires there also. You're looking for less than 0.4 volt difference from the alt BAT terminal to the positive battery post. Same goes for the ground side.
Make sure the battery cables themselves are in good shape. Try not to use anything less than 2 AWG cable. I know parts stores carry 4 & 6 AWG, 4 is better than 6 if you must.
If your mechanic is knowledgable enough, he can increase the voltage setting on the regulator, providing it has screws on the cover for removal.