heres an odd one for ya.....

You can temporarily short across the ballast resistor with a wire to see if it changes the behavior. The reason for the ballast resistor is to lower the current draw through the points and the coil. This makes them last longer. The voltage is not really that important, and it can be different from one car to another depending on the coil and resistor used.

Something else that I haven't seen mentioned is vacuum advance on the distributor. Do you have it hooked up? If so, to what port on the carb? When you set the timing, did you disconnect it?

Another thing you may want to do if you haven't already is run some cleaner through to remove carbon build up on the pistons and combustion chambers. Something like GM Top Engine Cleaner or Seafoam.