Tex........the original N-ammeter was a "loop" feedthru (just stick the wire thru the loop) Modern replacement ammeters are 2-terminal and you must "cut" yer power wire to fit on the terminals. tip: iff'n the meter reads backwards, just exchange the 2-wires. Simple, eh?
Surprizingly enuff, ammeters don't care 6-or-12V, and really don't care positive (+) or negative (-) ground. But you care. Just as long as the meter needle show charge (+) when engine is running.
Don't sweat the ammeter full-scale reading, yer 6-volt genny ain't never gonna output 20-amps. (some 12V alternator can pump 135-amps, whoo-whoo) .......Dell, a 12v advocate for the right reasons