1968 with no power in cab

[RustyPile]

Sometime, when connecting the inner and outer (inside cab to engine bay side) halves of the firewall plug, the terminal(s) will get pushed back in the connector slot.. Take a look again at those terminals and make sure one of them isn't pushed back and just not making contact.. As a clue, when this happens, it's usually the male terminal..

{EDIT}
I forgot to address your ammeter question.. The stock factory ammeter and its circuit isn't an "active" part of the system.. Any part of it can fail and will not affect the performance of the electrical system whatsoever.. The ammeter circuit is referred to as a shunt circuit.. Meaning it only reads the difference in voltage between 2 widely separate points on the main power distribution wire.