Voltage drop

I have a 1977 Firebird with a Chevy 305. The curb idle speed for this vehicle is 500 RPM. When the car is in park the Voltage is up at 14.4 Volts. When the car is put into drive the Voltage drops to around 13+ Volts. If the headlights are turned on the Voltage drops to 12+ Volts. I have had the alternator tested and it shows good. ( I have replaced it with a spare and still the same ). The battery is alomst new and both terminals are clean. I have checked the ground at the alternator bracket and it is good too. If I hold my foot on the brake and rev up the engine to around 800 - 1000 RPM the voltage climbs back to 14.4. Is it normal for the Voltage to drop this much or is there something else that could be causing it? Any ideas would be greatly appreciated.

The battery should stay up near 13.5. Check the other accessory pulleys and make sure one's not binding up on you.

There is only three things on this belt. Alternator, crank and water pump. I will check to see if water pump is the problem.