Steve, excellent question, let an engineer provide you with assistance.
1) First of all the decision to upgrade to an electronic ignition is YOURS ALONE, none of ours, so I will dispense with a discussion of the advantages or disadvantages.
2) REGARDLESS how your tractor was original, as a past tractor dealer I saw literally hundreds that were modified over the years ESPECIALLY regarding original POSITIVE Ground or NEGATIVE GROUND plus the use of external ignition ballast resistance devices when a 6 volt coil was used on a 12 volt tractor, many of which went bad and were removed or replaced or by pass jumped around and perhaps a substitute 12 volt coil was used. I CAN NOT KNOW WHAT YOU NOW HAVE?
THEREFORE IT IS IMPERATIVE FOR YOU TO DETERMINE IF:
A) YOUR TRACTOR IS NOW POS OR NEG GROUND ?????????????
B) WHETHER OR NOT THERES A BALLAST RESISTANCE DEVICE BETWEEN THE IGNITION SWITCHES IGN OUTPUT AND YOUR COIL INPUT ??????????????????
C) WHETHER OR NOT YOU HAVE AN OPERABLE BALLAST BY PASS SYSTEM
I JUST SAW SOOOOOOOOOOOOOOO MANY TRACTROS MODIFIED OVER THE YEARS
Based on your post, there?s a chance you still have a ballast (since you read 10.5 volts instead of 12 if all was well and points were closed, UNLESS YOUR BATERY IS LOW OR YOUR METER INACCURATE) but that 6 Ohms across the coil seems inconsistent with what I would expect ??????????????
!!!! YOU MUST ORDER THE PERTRONIX TO MATCH YOUR GROUND OR YOU CAN FRY IT !!!!
NOW FOR MY ADVICE
The above being said, and to get the most bang for the buck and reap the benefits which an elec switch has to offer, I WOULD CHOOSE THE AFTER MARKET HIGH ENERGY COIL PERTRONIX RECOMMENDS (such as their Flame Thrower) THAT?S SUITED FOR AND IS COMPATIBLE WITH THE SWITCH YOU USE !!!!!! The thing is even with an old stock coil of the correct voltage and resistance, sure you still get a spark and reap the advantage of rapid positive switching versus ringing bouncing burning mechanical points, HOWEVER if you upgrade to their switch and a properly matched (Voltage and resistance) high energy coil, you can end up with more spark gap energy !!!!!!!!!!!!!!!
While you can still use an existing ballast if you still have one but that would determine what coil you use I WOULD REMOVE ANY OLD BALLAST (if it still has one) and you can get by without the use of any ballast by pass such as when they used 6 volt coils on 12 volt tractors,,,,,,,,,,,,,,I would order a Pertronix elec switch such as their Ignitor that matches your ground,,,,,,,,,,,,,,,,,,,,I would talk to Pertronix and buy a new High Energy Coil such as their Flame Thrower (THEY recommend NOT any of us here) that?s suitable for and compatible with the switch you so choose,,,,,,,,,,,,,,,,,,,I would likewise talk to Pertronix regarding the choice of plug wires as for many units they do NOT recommend wire core plug wires.
SURE you could use any existing ballast resistor and SURE you could use an existing coil of the correct voltage and resistance and SURE you could still use a 6 volt coil and a start by pass system BUT IF YOURE UPGRADING TO AN ELEC SWITCH WHY NOT USE THEIR RECOMMENDED HIGH ENERGY COIL AND DO AWAY WITH ANY POSSIBLE OLD BALLAST PROBLEMS (Including any start by pass system) ????? START ALL NEW
BOTTOM LINE, Use the coil Pertronix suggests that matches your switch (THEY will recommend the correct voltage and resistance of the coil) and ask them about the best plug wires that suit your coil and switch???..
Hope this helps: Determine your Ground,,,,,,,,,,,,,,,,,,,,,Determine if you still have any ballast device,,,,,,,,,,,,,,,Talk to Pertronix to choose your switch and coil and decide whether or not to still use your ballast if it has one (but that affects the voltage of your coil remember, I would toss it and use a full 12 volt rated Flame Thrower coil instead).
Best wishes, post back any questions (better yet ask Pertronix) God Bless and Happy Veterans Day
John T BSEE, JD Retired Electrical Engineer