Battery shorting killed all power
Its hard to follow when you write killed power but everything in the engine bay is OK.
Tell us how you determined things are OK, and how you determined there is power at the bulkhead.
The 1975 main power circuit is a little different than other years.
If you are using a voltmeter or test light, check for voltage at the bbattery positive and the alternator output stud.
That will indicate if there is a break in the main feed between the battery and alternator.
At the bulkhead, if you can, check if there is power at positions J, then T, and Z.
You may not be able to easily because in my experience these terminals have rubber seal on the engine side connector.
Its hard to follow when you write killed power but everything in the engine bay is OK.
Tell us how you determined things are OK, and how you determined there is power at the bulkhead.
The 1975 main power circuit is a little different than other years.
If you are using a voltmeter or test light, check for voltage at the bbattery positive and the alternator output stud.
That will indicate if there is a break in the main feed between the battery and alternator.
At the bulkhead, if you can, check if there is power at positions J, then T, and Z.
You may not be able to easily because in my experience these terminals have rubber seal on the engine side connector.