Ditto to what TMP has just said.
HEI are infamous for burning a hole right through the rotor, so the spark goes directly from the coil to the distributor shaft.
It takes the path of least resistance. If there is corrosion build-up on the dist cap terminals, or bad wires, or bad plugs.
I'd also recommend the better quality cap and rotor with BRASS rather than aluminum contacts.
If the cap and rotor don't fix your problem, and you still have intermittent or no spark ... the ignition module is a fairly common culprit.
If you replace that, be sure to use the dielectric compound which is (or should be) supplied with a new module.