voltage not even

[macdodd127]

I just replaced the engine in my '69 C10. Before putting the new engine in, we put on all the pulleys and also a newer alternator. All run off of a serpentine belt now. Question is, with the newer alternator, it surges back and forth at 12 to 14 volts at idle or in gear. Once the engine is revved up, the voltage is consistent at 14 volts. I'm wondering if the voltage regulator needs to be updated to one that is matched to the higher amperes of newer alternator? Of course, with the head lights on at idle or in gear, you can see the surge,,brighter/dimmer. Thanks Bill