What is the item that you are cooling the "Head Unit"??? Please use the proper terminology such that appropriate advice can be issued. It sounds as though the Alternator is good but the wiring is faulty and not connecting the alternator to the battery properly. Do you have the wiring diagram for the 240Z? What is the "Digital Gauge"??? What does this gauge indicate? The 240Z has the ammeter in series with the alternator and the battery to measure charge/discharge current. What is the ammeter indicating throughout all these exercises? An appropriately charged battery should be slightly greater than +12 volts. When cranking the engine for starting the voltage should not go lower than 8 volts DC. If it goes lower there are bad cells in the battery. When the alternator is initially charging after engine start, the alternator voltage should be +14.x volts. As the battery attains a charge the alternator voltage should taper off somewhat from +14v yet still stay above +12v. The newer alternators need only a small amount of current from the battery to provide excitation for the alternator field to cause it to generate 12 - 14 vdc. There may be an excitation return (2 wire excitation) to ground or battery negative to complete the circuit. Normally the case of the alternator is the return or ground path for both excitation and heavy current to charge the battery. The other heavy post or connection on the alternator connects back to the battery through a heavy current (low or no resistance to current flow) fused wire. Older alternator charging systems had more (complicated) wiring than newer charging systems.
Hope this helps. Will look at the 240Z wiring diagram to see which OLD alternator system wiring IS NOT NEEDED.
Source: Suddenly not keeping a charge.
This is EXTREMELY interesting. Would like to do the same and also fab a duct with a smaller opening to the radiator. The desire is to get aero such that my 240Z is stable at higher speeds and also get VERY good cooling of the radiator.