Wiring harnesses

1976 wiring is very different than '74
Can't tell you how different '75 is from '74.

Radio shouldn't be a problem. Look in your cars fusebox and see if there is a fuse or at least a fuse holder for the radio. That would be the simplest place to power it from. For the light at night it will also need a power wire spliced into the instrument lighting.

For a 60 amp alternator, I think the easiest is add a parallel feed from the alternator through a grommet. A little more work is to copy the factory option for 60 amp alt on '73 cars with rear defroster grid. Not sure what was done in '75.