FIRST, verify that the rotor turns when the starter motor is cranking the engine. If the roll-pin on the distributor gear shears, the distributor may not be turning, or not turning smoothly/steadily.
If the distributor rotor turns...
Get a spark-tester CALIBRATED FOR HEI, a spark plug boot from the Treasure Yard, and a jumper wire. Cut the boot as shown. (Photo 1.)
There are many styles and brand-names of spark testers. My favorite--and the GM-Recommended type--is this. There are other brand names of the same sort of tester. NOTE: A "spark tester" that make a light-bulb glow is WORTHLESS. You want one that forces the spark to jump a calibrated gap.
http://www.amazon.com/dp/B003WZXAWK/...=2VLYZKC3HBBDO
https://www.amazon.com/dp/B003WZXAWK...=2VLYZKC3HBBDO
Flip the distributor cap upside down, secure it away from the distributor rotor. (DO NOT disconnect the wire harness from the cap.) Install spark tester as shown,
ground the jumper wire. Turn ignition "on". (Photo 2.) Crank engine. You should get consistent sparks at the spark-tester. If you do, suspect a failed rotor, failed plug wires, or maybe a failed distributor cap. If you do not get sparks...
...Connect the lead of a 12-volt
INCANDESCENT test-light to BATTERY POSITIVE (or the alternator output stud, if that's more convenient.) Touch the pointy-part to a good ground, assure that the test light lights up. Remove the pickup coil connector from the ignition module. Touch the pointy part of the test light to the "G" (Green wire, small) terminal of the ignition module. The test light won't light-up. That's OK. Remove test light probe from "G" terminal. (Photo 3)
GM has many "HEI" modules. Perhaps you have something other than the usual "4-pin" module. If so, you'll have to identify which pickup-coil terminal on the module to probe. (Photo 4--Not a complete listing of every HEI module.)
Every time you REMOVE the test light from the terminal, you should get a spark across the spark-tester. If you do...the pickup coil has failed. If you don't, it's the ignition module, the ignition coil, the wiring connecting them, or some combination.