Slant 6 No Spark
Maybe. But hopefully he got your attention.
When the ammeter is pegged in either direction there is problem! A big problem!
Knowing its a 68 with an ECU and a points type regulator suggests it may have been miswired when the ECU was added.
Could also be a battery problem - internal short - or something else that was full fielding the alternator for long periods of time.
Lets go back to why a pegged ammeter is big problem.
If you can picture current flowing like a water, a few amps is like a creek. In contrast 40 amps (pegged) is like fast moving river. But the wiring is only the size of a stream bed. You can imagine what destruction happens when a river like torrent flows through a stream bed.
The ammeter shows whether current is flowing in or out of the battery. Unless its been miswired, that's all it shows. In other words, it shows the amount of Battery Discharging or Battery Charging.