help same old voltage issue but…

1966 dart 273 2v tried it all electronic distributer, electronic ignition, and voltage regulator, alternator 78amp..was the biggest in stock, 2 new batteries, cleaned all grounds and the bulk head. bypassed gauge voltage starts at 13.5 and drops to 12.5ish in less than one minute ..any other ideas... when put in drive, kills itself. all help is appreciated
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