The pulley looks normal size to me but I could be mistaken. It's not abnormally large if that helps. Also, the rpms aren't dropping when it's hot they actually are going up a bit and the problem is even there when it's in park and idling higher than when in drive, just not as bad. For example normal charging is about 14 volts in park but when it's hot is only about 12.8 at the same rpm. Something is getting hot and reducing the charge or limiting the charge. I have never changed the voltage regulator and looks like it's from 1971.
