A dash ammeter is about as accurate as sticking your finger out the window for a speedometer. It's just a glorified charge/no charge indicator. Does the needle sit in the middle of its range with the engine off, and does it deflect in the (-) direction when things like high beam headlights and the heater blower are operated with the engine not running? How was the regulator adjusted when it was replaced? There are three mandatory adjustments that need to be made- - - -closing voltage, current regulator setting, and voltage regulator setting. Just buying a regulator and bolting it on has about a 10% chance of working properly. The shop that rebuilt your generator should have been able to match the output of the generator to the voltage regulator, if they know anything at all about the old charging systems. That's the way I've done it since the early 1960's.

With a fully charged battery, you should see 7.5 volts at the battery posts with the engine at 1500 RPM or so. Your 6.8 volts is WAY low unless you have a seriously discharged battery.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!