What is the max alternator amperage that can be used on the stock (Mustang II) wiring?
This is a particularly tricky question that came up on social media;
so, i'm going to throw down some fundamentals.
The equipment ( stereo, horn, ignition coil, forward lighting etc.) dictates the loads.
The loads dictate the size of the conductor. 55W/60W headights will be on 16 or 18 gauge wires unless you have a aero-nose fox mustang with the stupid 110 W bulbs. A/C compressor clutch wiring is heavy. rear window defrost are 10 or 12 gauge.
The circuit protection (fuse, fuse link, circuit breaker) protects the conductor and not the equipment. Don't ever put bigger fuses in when you increase the loading (until after you change the conductor). the wires will get hot, and insulation will melt through and it'll hook up with the cool conductor next to it and light up that circuit. The worst harness I've repaired so far had all 20 amp fuses in every slot in the fusebox and all kinds of melted garbage to repair.
The voltage regulator controls the alternator output voltage for the charging circuit - the battery and the car will not draw more amps than the equipment will require. if the voltage regulator fails your alternator may make 0V or 20V. if 20V happens, the fuses will probably blow from over-current before the wires get hot.
This question is tricky but shows some missing fundamentals. Stock wiring can tolerate any regulated alternator or generator with the correct voltage output. Stock wiring can not tolerate equipment load increases of or over 20%. Stock wiring will be protected by the correctly sized circuit protection.
Fun Anecdote - i have a '57 Mercury pick up with 99% STOCK wiring and a STOCK 12V negative ground system. I took out the 38 Amp generator and installed a 120 Amp 3G alternator out of a 95 mustang. i used the Fusible link section out of the mustang harness to protect the wiring and it was super easy. I added a fuse and a relay to run the TFI ignition module and coil - also protected - also perfectly fine. zero issues - nothing melting.