Slant 6 HEI problems

It certainly sounds like the spark is getting weakened through the distributor. OP, measure the resistance through all of the spark plug wires and coils wires. A few thousand ohms is fairly normal for most wires; it will be higher for longer wires. Anything near or over 10k ohms ought to be tossed.

Look under the cap and see where the rotor is pointing when it fires, to see if it is pretty close to the spark tower contacts on the inside. And measure the distance from the center of the rotor to the tip and make the same measurement on the diameter of the contacts on the spark towers. Clean and dry the inside of the distributor cap and look for any carbon tracks or trails. Read the links in post #3. Is this the same cap you were using before the HEI?