You're probably got the ammeter shunt resistor going into meltdown. The original '58 generator was about a 30 amp so the shunt was sized for it. When you're pegging the ammeter the shunt is getting extremely hot and may be the cause of your failures. You may want to try bypassing the ammeter by tying the two wires together with a small screw and nut, tighten it good and tape it up good. Then see if your alternator problems go away.