Screaming belt after switching out the alternator

Sounds like you may have a couple issues going on here. You may have a bad water pump and it sounds to me like your belt is way too long. If we are talking about a '65 Dart, you only have one belt for the fan/water pump and alternator. If you're lucky you have power steering but that will be on a separate belt.

Take off your current belt. If the water pump pulley is as easy to turn by hand as the alternator, you need a new belt. If the water pump pulley does not turn freely, you need a new water pump.

To find the proper belt length, set the adjustment on the alternator tension in the middle of its range with the belt off and the bolts snugged down. Run a piece of string, parachute cord, det cord, etc. around all the pullies like you would a belt. Pull the string tight and tie it together with a knot. Cut the string and stick it in your pocket. When you get to the parts store pull the string out of your pocket and tell the counter guy you need a v belt that long. Over 50 some years catalog listings get garbled but if you know how long the belt is that you need, a competent counter man can fix you up.

Thanks for putting up with Ft. Jackson for us. It really sucked 42 years ago and I doubt it's much better now.