With the ignition "ON", points closed, WHAT voltage are you seeing on the primary terminal on the coil that is connected to the ignition switch... if full "12 Volt" battery voltage, you need a coil with a primary resistance in the 3 - 4 Ohm range.
If the voltage measures in the 6 - 8 Volt range, that indicates there's a ballast resistor in the circuit and you can use a coil marked "Use with external resistance".
These voltage readings ASSUME the points are closed and making contact AND the primary of the coil is not "open". In other words, the primary circuit must be intact and flowing current or the voltage measurements will be meaningless.
As to the secondary resistance, it will likely be in the 6000 Ohms to 12,000 Ohms range, measured from either of the primary terminals.
(There will be a tiny variation in the secondary Ohms reading depending upon from which primary terminal the measurement is made 'cause one way the primary resistance will be in series (added to) the secondary resistance.)
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Today's Featured Article - 12-Volt Conversions for 4-Cylinder Ford 2000 & 4000 Tractors - by Tommy Duvall. After two summers of having to park my old 1964 model 4000 gas 4 cyl. on a hill just in case the 6 volt system, for whatever reason, would not crank her, I decided to try the 12 volt conversion. After some research of convert or not, I decided to go ahead, the main reason being that this tractor was a working tractor, not a show tractor (yet). I did keep everything I replaced for the day I do want to restore her to showroom condition.
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