Did you install new plug wires with the HEI? If not then I suggest a set of "sprial core" magnetic suppression wire - way better than carbon core.
Also make sure that you use silicone grease on the inside of all the plug wire boots.
The added zap from the HEI has a tendency to eat old carbon core wires - the carbon breaks down inside the wire and the required voltage to jump the gap goes up which makes matters worse.
