What alternator is on it (stocker, 3G, etc.)? Are you serpentine or v-belt? Any wiring changes like 4 ga from the alt to the battery or battery to chassis and chassis to block? Need to know a lot more about what you have to start chasing stuff like this.
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?
Man I’d put the battery on a charger and have a parts store load test it.
Could be a bad ground or 12V+ cable so check the connections. If you are not running much else draw wise I would think the stock style alternator would be fine.