Wiring harnesses

All I can add is to remember that the alternator 'rating' is approximately maximum possible output.
The actual output in operation is only what is demanded.
On A-bodies, the optional 60 amp alt wiring seems to have first been implemented 1973 MY with the defroster option.

Another thing different by 1976 is at least two power feeds through the bulkhead instead of one. That may have been done before '76 too. IIRC there is a similar division of the power feeds in the steering column connection earlier in the 70s.