There is one advantage when you keep a 6 volt coil with the external ballast on 12 volt conversions, as follows:
With the 6 volt coil and EXTERNAL ballast, ITS POSSIBLE TO BY PASS THE BALLAST WHEN CRANKING FOR A HOTTER START SPARK WHICH CAN IMPROVE COLD WEATHER STARTING. However, if you use a 12 volt coil YOU DONT HAVE THAT OPTION as theres no external ballast you can temporarily (while cranking) by pass. The ballast by pass system applies unballasted battery voltage to the coil while cranking, which can be good beacause while cranking the battery voltage drops which can reduce spark energy WHEN ITS NEEDED THE MOST.
HOWEVER that requires use of a Ford type starter solenoid with the extra lil I terminal (See Jims excellent post below) thats used to by pass the ballast. Or else a saddle mount on starter mechanical start switch (like Deere uses) with the extra terminal used to feed the coil (by passes ballast) when the start switch is depressed.
BUTTTTTTTTTTTTTTTT if you just have a simple dash/pedestal (NOT on starter) mechanical push to start switch (NOT any electrical solenoid) THEY DONT HAVE THAT FEATURE in which case theres no advantage to the 6/12 and by pass system so you may as well just use a 12 volt coil.
SUMMARY for improved cold weather starting on 12 volt conversions, a 6 volt coil with external series ballast COUPLED WITH A BALLAST BY PASS CIRCUIT/SYSTEM is great BUT it requires EITHER a Ford type starter solenoid and associated wiring ORRRRR a mechanical saddle mount on starter start swtich (which I dont think your M has) but if its only a dash/pedestal push mechanical switch that by pass isnt available.
ONE OTHER OPTION IF THERES NO SOLENOID OR SADDLE STARTER SWITCH but you still want a 6/12 system with improved cold starting, is to use a temperature compensating (Thermistor) link to serve as the ballast and it sort of by passes the ballast, cuz when its cold its low resistance but once warm its like 3 or so ohms so you can use a 6 volt coil on a 12 volt system,,,,,,,,get better cold starts,,, no extra solenoids or by pass systems or wiring to fool with, you can still use a mechanical push to start switch ITS REALLY A GOOD SYSTEM !!!!!!!
FINALLY if you have a good engine and its well tuned and your ignition systems in top shape and you have a good heavy battery n big cables, HECK YOU PROBABLY DONT EVEN NEED ANY 6/12 OR BALLAST BY PASS ETC......Id just use a 12 volt coil n be done with it no ballast or by pass worries, MANY TRACTORS ARE LIKE THAT N WORK FINE
If I missed any options (was in a hurry) hope the other fine gents will add them, unless you need to start at sub zero temps I wouldnt worry n just use a 12 volt coil........
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Today's Featured Article - George's Fordson Major - by Anthony West (UK). This is a bit of a technical info to add on to the article about George's Major in the "A Towny Goes Plowing" article. George bought his Major from a an implement sale about 18 years ago for £200.00. There is no known history regarding its origins or what service it had done, but the following work was undertaken by Harold alone to bring it up to show standard. From the engine number, it was found that the major was produced late 19
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