Billy the kid,
First lets start with: "Well I have power at the coil on start and run. I tested with a test light. I guess I need to put a volt meter on the coil input wire to see what I get.
When I jump the coil the old girl will run."
When you jumped the coil, You supplied power. It must have not had enough power already!
"Why would you need a resister with a 6 volt coil
but I can replace with a 12 volt coil and not need one?"
Long story short. You need a certain amount of current to run through the circuit (as in Amps) to create a spark. Amps equals volts divided by resistance. The problem is volts are different when you start vs when running. Maybe 8 volts left when you crank the engine. Around 14 volts when the engine is running and the charging system is working.
So the old way to do that on a 12 volt system was to run two wires to a low resistance coil (called a 6V coil). Wire number 1 came from the starting circuit, since the voltage is low when starting, no additional resistance is needed. Wire number 2 comes from the running circuit, you need resistance to limit the current, so they add a ballast resistor. In both cases the current is enough to create a spark.
Now the coils (called 12 volt) can be placed directly in the circuit and are able to produce a spark and limit current under both the start and running conditions with out additional resistance. These coils are marked "No resistance needed".
I hope this brings to light some of the info you need.
JMOR or several others can explain more if you need it. Just let us know if this is enough.