I was feeling better, now I can race again and the Pittsburgh Mopar race is coming later this month. So, I took a break and went in the house (air conditioning) and called MSD to discuss my problems.

Vic said the Delco 1-wire alternators are the worst for interfering with the MSD box. And the Chrysler factory magnetic pickup acts like an antenna. With my big block, that alternator is right next to the distributor! He recommended the shielded pickup cable and to test by wrapping the wire with aluminum foil. I told him I already have the shielded cable, but there is about 8" of exposed wire coming out of the distributor because I have an adapter cable to go from the Chrysler connector to the MSD connector. So I will try wrapping that part.

I went out to the garage and wrapped the unshielded part of the wires and ran the engine. It didn't change, OK with fan off, not with fan on. By now I am sure it is alternator interference, and I figure the more amps the alternator is putting out (fan on), the more interference. So I tried wrapping the alternator in foil, and that was futile...

I decided to modify my distributor to get rid of the unshielded part of the cable. The MSD cable now plugs right into the distributor. The phillips screw locks the vacuum advance plate since I removed the vacuum can.

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And now it fires perfectly even with the fan on!
Man this electrical interference thing is a pain, but it is now fixed. Here is a pic of the installation in the car: