How do you intend to wire it? ie what size wire from alternator (and other loads) to battery, and how will the alternator be connected? I'd consider not using the stock ammeter...but I do have them in a couple of my late 50s chevy trucks with 10si alternators, and haven't had any issues. One I've been driving that way since 1980.
What's the amp out put of the Alternator ? What's the amp gauge rated for ? Chances are the alt. puts out more than the gauge is rated for. Done wrong you had better have a battery shut off or good insurance.
What kind of gauge is it? Does it have two connections on it, or does have a 'loop' that the wire p***es through? What's maximum current rating according to the dial? The loop type doesn't have a direct electrical connection, so will be more forgiving. And most important, are you adding any additional electrical loads that weren't on the original car? If you're not adding any loads and it's a 'loop' type, it should be safe to use the original gauge; because there's no direct connection, if the charge rate exceeds the gauge range, at worst the gauge may fail. If there is direct connections on the gauge, it becomes much iffier... Now a gauge failure can result in a fire. And installing a shunt just isn't practical for a home mechanic, as getting reasonably accurate resistance values for the gauge and shunt will be beyond our capabilities. And shunt-type ammeters when used by the OEMs haven't proved to be very reliable.
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