If its a regular 720 gas tractor, it used a 6 Volt Coil and the external series voltage dropping (12 to 6) Ballast Resistor.
Therefore the coil you need may be labeled "6 Volts" or "12 volts for use with external ballast resistor" or "12 volts requires external ballast resistor" etc the last 2 of which are in reality more like 6 volt coils since the ballast drops 6 leaving only 6 on the coil
If it were me Id keep it like original and use the ballast plus a 6 volt coil for improved cold weather starting (assumes the ballast starting by pass circuitry is in place and operative) . If you want to toss the ballast then you would have to use a full 12 volt rated (NO Ballast) coil and do some minor rewiring.
That 40,000 volt coil you mentioned given the same conditions wont fire at any higher voltage then stock set up, its just that it has the capacity, if required, to achieve higher voltage if necessary. Its more for a system with an elec switch and wider plug gaps etc where higher firing voltages are used. Depending on its internal resistance, if its much less then stock and you still use points, it could cause premature points burning and not yield any more performance whatsoever. Id stay with a stock 6 volt coil unless you wanna fully upgrade to an elec switch and a matched high voltage higher energy coil and run a wider plug gap which can improve starting and idling charactersistics somewhat but may still not add much if any at all HP