I have built quite a few 12 volts car voltage regulator for alternators with great success, and are still in use 10 years later.
I built hem up in a separate box which i mount on the fire wall.
The wiring goes to the 2 field connectors of the alternator.
The excitation goes as normal via the dash alt. ignition lamp, regulating is done via a zener, two transistors and a 2N3055 power NPN transistor.
It is good practise to replace brushes and check the bearings of the alternator as well.
It was not a cost issue but the challenge for me to built and use this circuit.
Can't see a safety issue here because the car battery will avoid a car stalling with no immideate warning anyway, the alt. ign. light will come on in case a fault occurs.