Most, if not all, of the harness suppliers, AAW, painless, Speedway.......do not recommend an ammeter. Several do mention the ammeter and how to wire it even though they don't recommend it. It is done the way I described. However they say the ammeter rating needs to be greater than alternator output. They also say a 45 amp alternator is max. It seems they don't understand that an alternator will not produce more than the demand. Therefore a high amp alternator on a older stock vehicle with stock electrical components will do no better than the OEM alternator. Which in your case an original was 42A or 55A in 1966 or a 63A in 1974......and now unknown. Maybe you now have 100A. When I knew you had no non stock electrical, my opinion was that it's OK to use the stock ammeter. It has a shunt. I also described how to wire it. It will have a fuse. However, I do not know the rating of the Mega fuse. Seems like 175A. That will not protect the ammeter. But it never was protected. The only info I can find on their Mega fuse is 175A. That makes no sense. It's too big to protect anything. Looks like two fuses, between the battery and the alternator......and between the battery and the fuse panel.
The harness makers are trying to be clear of liability. They don't know what ammeter may be used, they don't know what components will draw how much current from an unknown alternator output rating. They know they are dealing with novices. SO they error on the safe side. I tried to analyze your specific situation and give an answer. You are on your own to decide what to do. To wait for other member's input or do nothing. I have no more info or comment on other ideas you are presenting. I give no guarantees on my suggestions.
This is why I like volt meters or idiot lights. Simple. You can now see why this always comes up during 12V conversions and harness wiring and electrical gauge questions. Ammeters cause a PITA for NO REASON. They are unsupported. They are no longer a technology worth playing with. Some old timers and purists love them for nostalgic or other unsubstantiated reasons. They don't cause a fire per se.
If you are unsure, and I don't blame you, Just get her wired and driving and worry about the charging monitoring later. Wire it per the instructions. Let the volt meter wires hang. Or install a cheap volt meter under dash. FYI: Ammeter needle is centered when off, Volt meter needle is pegged left when off.
Greetings from The Peoples Volt Meter Factory.
Crick
