Yeah if your going to switch out to a new alternator you really should get rid of the ammeter and get yourself a decent Voltage gauge.

 

Not true. The ammeter is wired in parallel with the main Alt-Bat feed cable. It carries a very small amperage and only measures the deflection in the system.

http://www.midnightdsigns.com/Mustang/66circuit.jpg

Note in this original wiring harness diagram that Wire 37 is the main alt-Bat feed. the gage is in parallel

 

I would think it would be what the shunt would hold. Anyway, if it was me, I would loose the Amp gauge and install a Volt gauge.

 

I have a 160A 1-wire alt & still have the factory ammeter gauge.
I run a fan, 1500W stereo, lights, etc all the time.
Like said above the Ammeter circuit is in parallel with the battery wire. Hardly any amperage is actually going through the gauge. If all the car power had to go through the gauge you would have a huge size battery cable running to the gauge cluster.

I also would like a factory appearing volt meter to replace my ammeter, but I don't want to replace all the gauges. If I find one I'll post it here.
I have found a replacement all digital cluster ($500) & this http://www.cjponyparts.com/NewTechGS.aspx
if you want to replace all the gauges.

 

It might just be me, but to me it looks like the ammeter is in series to me,,,,,

I don't really feel like studying the diagram in anymore detail, but the way I have always understood ammeters, in order for them to produce reliable readings, everything must pass through. They measure volume and direction of current flow..... I don't think a primitive parallel circuit will work for this....

 

http://www.midlife66.com/wiring/66ignit.jpg
Here's the whole diagram. If you physically look at 655 and 654 in a 66, they are 18-20 gage wire. Line 37 is the 8-10 gage wire running from the back of the alternator to the positive post on the solenoid. All the ammeter does is measure whether the alternator is charging (+) or if the battery is draining (-). It can't carry the full amperage of the alternator's output.

 

They will work one of two ways. There "will be" a shunt somewhere to measure the resistance. On a parallel setup, the danger is when the primary circuit fails and the amp gauge becomes the fuse, then you will have a fireworks show and may even toast your car. You can run a thousand amp alternator and giga amp stereo and everything will be fine untill you have a failure. These old systems were designed for maybe a 65 amp alternator at most, and the shunt will be sized accordanly. It will only measure what goodies the car pulls and what the alternator is pushing. Updating the electrical system to higher amp alternators, high watt stereos capable of sucking a thousand watts (83.3 amps at 12 volts) halogen or additional lights, high draw electric fans, etc, can cause things to overload fairly easy, especially when everything is running wide open and the system is pushed to its limits. Soooo, if you are going to make these upgrades and "JUST" have to have an ammeter, I would leave it out in favor of a voltmeter, and eliminate the possibility of a future problem or fire. On a side note, the ammeter will stay hot all the time (when the battery is connected) and is "NOT" protected by a fuse or fusible link. Fusible links started to appear around 1969. This is an easy fix by installing a 20 to 30 amp fuse or link and a 12GA wire. Just my 2 cents.