First: check that you have 6 volts to the - terminal of the coil.
If so you need to investigate in the distributor. A few suggestions are:
Points. Are the contatcs clean and not badly pitted.
Are they opening and closing properly? anout a .015 opening on the high spot on the cam.
Do you get a spark at the points with the key on, and you manually open and close them?
As above? wires in the distributor. Another thing is if it has the big flat connector to the points. It must not touch the frame. There should be insulation around or behind it.