old dog,

It still sounds like ignition to me. I would check and/or think about replacing the plug wires....unless that was just done! Under a load, cylinder pressures increase. This means it's tougher for the spark to make the big jump.

If the ignition is marginal, it can fail under a load. It could still be a carburetion issue that dumps a bunch of fuel under load (low vacuum) and makes it difficult to ignite.

If the plug wires are old, that might be the place to start. I would also make sure the wiring to the coil was proper. If you had a HEI then you probably ran a full 12 volts. Did the point style coil you stuck on require this, or did it require a ballast resistor.

I have seen bad motor mounts allow the engine to torque over slightly under a load...just enough to tug on a primary lead wire and screw things up. This is a real long shot but it never hurst to keep your eyes open.

Good luck and keep us posted on your progress.

Stuart