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

Your load is on the downstream side of the ammeter, so it's going to read high when the amp is turned on.

Turn it off and see what the ammeter reads, bet it's closer to the midline.

This is a big issue with mopars and where auxiliary equipment power is sourced in the car. I WOULD NOT continue to run the amp with the OEM wiring in place, carrying that load. If you must run it off the battery, run a direct charge line from alternator charge stud to starter relay or battery +.