That's sucks. I just saw the other threads - sorry to read that.
'75 has two fusible links. But if the key will cause the starter to engage, they are both intact.
View attachment 1716293593
Follow the line from the battery to the key switch. When the key is in 'start', a connection is made to the yellow wire which then connects back to the starter relay.
The 'alternator' gage actually indicates battery charging or disharging. So while starting it show a little discharging.
Headlights and dome lights are on seperate feeds that branch off from a splice. So its hard to come up with a scenario where both of those fail to work and the starter engages, other than maybe a chassis ground.