Alternator Gauge Reading High With 300 Watt Stereo System Installed---1969 Electrical System

Hi everyone. I installed a 300 watt stereo system in my 1969 Valiant with an original electrical system and my alternator gauge is running high. It's reading about the 3/4 mark, between the center sweet spot and max. I just replaced a dead battery and someone told me, I could be boiling it from overcharging, due to my system. He recommended that I get an upgraded alternator with a higher amperage rating. Would this solve my problem without creating others? Is there anything else that I might try before replacing the alternator? Thanks for the replies.