YOUR QUESTION: So If you have a 60 with a 12 volt battery, you want to run wire from switch through a resister and into a 6 volt coil?
ANSWER: You could if you like!!!!!!! or else simpler to just use a 12 volt coil that way you dont have the wiring worry and extra connections and expense of a then un needed ballast!!!
YOUR QUESTION: If you buy a coil with a resister built in? You would want a 6 volt coil with internal resister and that would work fine as well?
ANSWER: SEE MY POST ABOVE, YOURE NOT GONNA FIND A COIL THAT HAS A STAND ALONE DISCRETE RESISTOR TUCKED AWAY SOMEWHERE INSIDE THE CAN
HOWEVER, You CAN FIND a "12 volt" coil orrrrrrrrr a coil labeled "12 volts NOT for use with ballast" orrrrr a coil labeled "12 volts NO ballast required" AND GUESS WHAT they are engineered and designed to operate at 12 volts nominal and like they say NO BALLAST REQUIRED
ALSO you can buy a coil labeled "6 volts" orrrrrrrr "12 volts FOR USE WITH ballast resistor" orrrrrr "12 volts requires ballast" AND GUESS WHAT!!! THOSE ARE ALL IN REALITY 6 VOLT COILS
SOOOOOOO on a 12 volt tractor you can use EITHER
a) A 12 volt coil
b) A 6 volt coil PLUSSSSSSS an external series voltage dropping (12 to 6) ballast resistor
Get it???? Its NOT really rocket science, post back any questions, hope this answers your great questions
best wishes and God Bless