Assuming no added devices other than your second tail light, your 12v system will have less amps flowing than the 6v system had. As you add additional load you may find that the ammeter needle momentarily hits the peg just after first starting.
Another thing you will probably see is a lack of charging (and maybe a discharge) when idling, especially with the headlights on. Your alternator was designed for an engine that probably idled at 750-800 rpm. I don't know the proper idle speed for your '46, but my '52 GMC idles at 450 rpm. My 10si alternator does not put out enough power at that speed to keep the headlights from dimming. I don't have an ammeter, but my voltmeter drops to somewhere under 13v in that condition. Increasing engine rpm just a few hundred rpm jumps the voltage back into the high 13s+.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
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