Ammeter Bypass a Must?

What I'm missing is that if the ammeter failed, the alternator may not be able to charge the battery. If true, you must fix that. The only path for charging current to the battery is thru the "+" terminal on the ammeter and that is a dedicated wire straight to the battery. The only exception is that a few special high loads in the cabin come straight off BAT+ without going thru the dash ammeter - convertible motor, police spotlight, ... .

As stated, the main purpose of the "MAD Bypass" is to take the high charging current off of the bulkhead connector terminals, to avoid melting the plastic. That happens when the terminals get corroded. Not a problem in 1963 and 1965 cars since they have dedicated buss-bars to feed those high currents thru [ALT (blk) and BAT (red) wires]. But, even with those, if you upgrade the alternator, it might output too much current for the dash ammeter to work safely. My cars all have the 1965 bulkhead (retrofit my 1964). I designed in a large diode pair which allows the ammeter to work normally at small charging currents (due to ~0.5 V forward drop of diodes), but starts bypassing the alternator output straight to the battery thru the diodes at higher currents (I measured ~0.75 V drop across ammeter at full scale). In effect, the diodes act as a "pressure relief valve" at higher currents (either direction, in or out of battery). Search for my post if interested, but don't fool with it unless you are an engineer or skilled tech.