not sure type of altenator just a factory type one? and serpentine belt yes.. worked fine for years never had this problem... before being parked for 2 years.
car does have hei dist swap and duraspark icm rmoved completely, push button and coil ran to toggles.
battery has not died no reason to suspect alt is completely shot, but would running the hei coil to battery create that much more of a draw? alt cant keep up maybe? need altenator upgrade?