what would be your diagnosis?????

Check the polarity on the distributor pickup wires!

There's a little note about it in the MSD instructions. If the polarity is incorrect, the MSD will advance or retard your timing like 40 degrees or something ridiculous like that.

I had a similar issue when I installed an MSD in my Duster a few years ago. The car would hardly start and when it would, it was missing, running rough, wouldn't stay lit etc. The only thing that was changed was the MSD. It was driving me nuts, everything was right.

I posted a similar question to yours on the Moparts race section and an old racer gave me the tip. Took a while for it to sink in, I did not realize what he was referring to at first. I went back and re-read his reply. Eventually I understood and gave it a shot and switched the polarity of the pickup wires - car fired right up and ran great.