[davepl]

I don't think you're going to solve this with an external coil. Even if it is the HEI upsetting the tach, it's going to be the ignition module (transistorized, maybe capacitive discharge not sure) that's throwing it off, not the coil.

So while you certainly can likely rig up an external coil to an HEI if someone makes the part you need, it's likely not a good idea and likely not going to solve your problem.

I had a tach that didn't work with my Pertronix, so I grabbed the "next one off the shelf" and it did work. Seems really hit or miss, and I don't know why.