Not sure what exactly you mean by "12V delco system on a 235" but I will assume:
1. You have done a 12V swap. Correctly. Battery, coil, VR, gen..... all 12V. Ballast Resistor and etc. or you swapped a 55.2 or newer motor.
2. You have one of those "special" 12V generators.
3. You checked battery voltage motor running (gen output) and motor at rest.
4. We are talking about a truck.
Since motor starts fine under a 250-450 Amp load. Since battery shows 12.6V. Since battery stays charged....... Nothing wrong with battery.
The 12V battery charger may be trying to take it to 14V+.
The truck Ammeter is reading current, as you know.
I suspect:
1. Bad ammeter
2. Bad VR
3. Bad Generator unless it shows 13-14V motor running
4. Bad wiring/miswire
5. 12V gen is putting out more than old ammeter limit. 1947 generator was about 35 amps max.
6. Possibly some kind of polarization problem
