Heavy Duty Alternator

The block in my 75 Dart Sport cracked during the freeze, the replacement 318 is out of a 78 Aspen and has the HD alternator, can I run this in a 75 Dart or do I need swap for the standard alternator on the cracked 318? Does anyone know the replacement cost of the HD alternator? The standard alternator is usually 40-60 dollars. -BD