Newer technology means precautions are needed, 12V system basically was set up for starting improvements...more reserve voltage to fire ignition while starter is off loading the battery surface charge...but once running, ballast resistor switches in to run ignition on 6V to preserve points during normal operating mode.
Further modernizing will require other changes of course, but the key is to remember that 12V systems are more efficient...requiring smaller guage wiring and ultimately lighter weight and more "reserve" power to run more AND COMPATIBLE modern accessories. If you are not building a 100% original machine, a hot rod with nostalgic looks but modern technology makes for a better daily driver...congratulations and cheers to a successful build.