I have run across some of those Proform distributors that the rotor phasing was so far off my HEI wouldn't make it to the right plugs at the right time.
Actually I have one here right now that I fixed by repositioning the pickup plate.
If you have one of these in this condition a regular spark would seem like the problem you have.
Autozone actually sells brand new electronic distributors pretty cheap, and is actually what I run my HEI ignition with and it works perfect.
I did recurve mine and limit the total timing with the FBO plate and Mr. Gasket spring kit.
One other thing.
That MSD might be sending the spark right through the rotor on a cheap one.
