Why would someone add an aftermarket ballast resistor to a 6 volt '37 Chevy MD field resistance lug on the headlight switch ?

The wiring of an ignition coil is fairly straightforward as there is only two terminals and regardless of whether there is an electronic distributor or the old fashioned contact breakers, in the basic 12 volt ignition system the wire from the ignition switch is connected to the + or Batt terminal of the coil and the single wire from the distributor connected to the - or CB terminal.

That system became quite rare over time as the ballasted system performed much better for little more cost. The primary resistance of the 12 volt coil is around 3 ohms and the ballast type around 1.5 ohms. It is important the correct coil is used.
You mentioned resistor so it is reasonable to assume it is a ballasted system. In this system one of three types of resistor is used - while many used a length of resistance wire integrated into the wiring harness the AC Delco system used a carbon "pill" type resistor mounted directly on the + coil terminal.
The third type is probably the best and is a ceramic unit with resistance wire inside, usually bracketed on the coil bracket or nearby.

The ballasted system uses a low resistance coil. One end of the resistor is supplied from the ignition switch and the other end is connected to the coil + terminal. There is a third wire directly from the starter solenoid also connected to the + terminal so when the ignition key is turned to crank the engine, battery voltage is supplied to the coil bypassing the resistor and producing a much more powerful spark making starting easier.

By 1986 many vehicles had begun to use either integrated distributors (with the amplifier inside) or a discrete amplifier mounted near the coil. The connections to these is fairly intuitive as multiway connectors take care of most of the wiring though the coil wiring remained basically the same on the + side.

If it starts but misses meaning it runs but not on all cylinders?.....then,
Are the spark plug wires in the correct firing order?
Are the spark plug wires in good shape?
Is there compression on all cylinders?
If a valve is bad it won't fire smoothly.
If you pull the plugs are any black with carbon?
(a black carbon coated plug will not fire,especially with a 6 volt system)
Is the distributor car attached fully and correctly?
When the distributor cap is off,is there any side play in the distributor shaft?
Is the point gap correct on the points?
Start the tractor cold and feel the spark plugs,the one or two that are colder than the others are the missing cylinders,if two are cold you could try switching those spark plug wires with each other,in case they are mixed up,and if two are missing are side by side you could have a head gasket leak between cylinder problem.
TIP 1, the BEST THING to do on a Ford 8N is to convert it to 12 volts neg ground.Add a alternator,(76 chev pickup alt fits good) leave the starter as it is (it does care if it's positive or neg ground or 12v) and add a 72 dodge ignition resistor in line to a 12 volt coil.(unless your new 12v coil does not need a resistor)
This set up will increase your fuel economy,add power and start instantly in any temp.
TIP 2,you can use unleaded fuel in a Ford 8n as it already has hardened valve seats.
BUT 10% Ethanol fuel is BAD,it can be used, but shut off the fuel at the tank and let the tractor carb run dry before shutting it off if you are not going to use the tractor with in a few days.

Many parts stores have loan-a-tool pressure gauges. What are you working on? Older cars have two wires to the ignition coil. On is a start only wire which is active only in the start position of ignition switch and bypasses the resistance circiut to supply 12 volts to the coil when starting. This is done to cause a higher voltage, lower amperage spark to assist in starting the engine. When you release the key, juice to the coil goes through a resistance wire (chevy) or ballast resister (mopar) to send about 6 volts to the coil to give a lower voltage, higher amp spark for more efficient fuel burn. That said, more modern cars don't use these 2 circuits.

I would say ballast resistor, this is a resistor used to reduce power at the coil from 13.8 volt down to usually 9 volt. this allows an engine to have extra voltage for starting and run on the lower voltage. ballast resister faulty it runs out of spark when you disengage the starter

indicates over voltage to the coils
failed resistor or problem in the ECM is allowing excessive voltage to the coils

Add more resistance (try 15 ohm resistor) until the blink rate comes comes down to where you want it.

The older chrysler products have a ballast resister on the firewall that is used for dropping the voltage to the ignition coil. During a start procedure, while the key is turned all the way in start position 12 volts bypass the resistor forcing a high output of coil voltage. When you release the key to the run position 12 volts goes ito the resistor & a lower voltage of around 6-7 volts comes out, & goes to the coil.
What happens, is the resistor breaks inside and no voltage is coming out the resistor. Thus only a start condition exist with no run.

The Multimeter has several functions.It test voltages [a/c, d.c. (+ plus, - minus)], then the RESISTANCE test to test 'ballast resistors". If the tester has a on-off switch, make sure that the scitch is OFF. If you can disconnect one side of the resistor, do so !! Place (+plus) side of probe on DISCONNECTED side, turn meter ON, place other side of probe (- minus)on other side of resistor.You should mesure the resistance you see written on the ballast. IF YOU CAN'T DISCONNECT ONE SIDE, just follow the same rules WITHOUT DISCONNECTING one side.
NOTE:: Rustyrooster, what Salkim08 said is true, but he simplifed it. Connected to the ballast on either side or both sides is a circukt (ckt.) that could effect the measurement.

1979 Chevy luv 1.8 everytime I turn on the lights running light fuse blows, everything else work, lights, turn sin.

If the car has a resistor coil with a ballast resistor to drop voltage from 12 to 8 then check the resistor wiring. When you engage the starter tumbler it by passes the ballast resistor and sends 12 volts straight to the eight volt coil to get good spark. When you release it the 12 volts goes through the resistor to 8 volts and then the coil. If there is a fault in this circuit the engine stops. Jump the 12 volts straight to the coil and start it. If it keeps going the ballast circuit is faulty. Paul