Your 12 volt coil will work if it doesn't have an internal ballast resistor. For standard, contact ignition systems, all ignition coils have approximately the same primary resistance, about 1.5 ohms, plus or minus a couple of tenths. Secondary resistance varies by manufacturer preference, but is not a factor with your coil as long as it is a Delco-Remy, or a replacement with the OEM specifications. High performance coils, heavy duty coils and those made for various electronic ignition systems vary in primary and secondary resistance, depending on the manufacturer and the ignition system. Some electronic systems even use the 'standard' coil- that is, nothing special, except a label.
I don't see where you should have any trouble with the 12-volt coil in your 6-volt system, as long as the 12-volt coil has about 1.5 ohms of primary resistance and secondary resistance in the range of 9000-10000 ohms; some variation is alright.
Harvester
