76 duster alternator charge wire gauge

That wire was 12 gauge from 1975 and earlier.
In 1976 it was changed to 10 gauge.
Be careful what changes you make in 76. I'm pretty sure it had an "external shunt" ammeter, but the actual wiring harness is what is used as a shunt!!! So if you go around modifying the harness, the ammeter may become non functional, damaged, or be grossly inaccurate.