Having not long gone through a similar exercise on one of my cars, which doesn't have a ballast coil.
Order of fault location would be,
1. Check the coil primary resistance if its 3 ohms then its not a ballasted coil, you need to dump the ballast resistor or fit the correct 6 volt coil.
2. Change the rotor arm for a red one.
3. Change the Dizzy cap for a good quality one.
4. Make sure the plugs are ok and gapped correctly
5. check or replace the ht leads the lead from the dizzy to the coil can very often cause weak spark when hot (especially if you are using a 12 volt coil with a 6 volt ballast resistor)
For me it was the "rotor arm" the quality of some of these after market parts has been very poor.
Have a look here http://www.distributordoctor.com/red-rotor-arms.html
Regards Mike