1968 Dart gauge help

Those cigarette lighter voltmeters are great. I use one on my newer cars (1996+) since they weren't nice enough to put even a voltmeter in the dash. Used to be $15 for just a voltmeter, so $5 w/ USB ports shows how clever those Chinese are (don't start a war to find out). It does not measure amps, just volts on the ACC feed. Toolmanmike is correct that there are clamp-on DC sensors one could put on the BAT cable in place of the dash ammeter, but they aren't inductive, rather "Hall-effect", and I doubt for $5. I use a Fluke one when I want to verify alternator output (cost $400 when we bought new at another company, I got one cheap on ebay).

Since the cluster is out, consider upgrading to an electronic adjustable voltage regulator, in place of that rectangular plug-in box. I bought several for $30 on ebay. Search "Plymouth" and "Voltage Limiter". The OE box is an old thermo-electric device (wax swells w/ heat or such) and not very precise, plus switches on & off to give an average 5 VDC output. That used to be inside the fuel gage, until ~1967 (until 1973 in Rallye dashes w/ tach).