I wish it was that simple. The cap has a mistake proof locating pin that allows it to go on only one way.

I had a bad MSD once. I sent it to MSD and they turned it around really quickly and the service was very reasonable. I dont think I had any spark though when it went bad.

A few more ideas:

Swap the MSD box for a known good one

Jumper 12v power directly to the MSD to bypass ignition circuit

Check valve clearances

Sounds like you have thought of everything, perhaps the ground wire on the MSD could be failing? Fully charged battery?

Check the plug wire sequence again, you might have them mixed up.

Check the timing, maybe your distributor turned, pull some timing out to get it started.

I really don't think it's ignition related, I think you have a fuel problem, pull all the plugs see if they are wet. Pull the tops of the carbs again and clean out the bowls and jets.

I'm no expert, but if a bad MSD could cause out-of-time ignition pulses, it could completely destroy an engine. This seems like a very unlikely failure mode. The fact that you're getting regular spark pulses suggests to me that your MSD is fine.

You can test for weak spark easily; just vary the distance at which you hold an empty spark plug wire from a ground. It's easy to shock yourself with this test, so either man up or take precautions. :) You are looking for the spark to jump a good 10-20mm.

I would not adjust timing, thus complicating your diagnosis, but make sure the dizzy isn't loose, check for shaft play, etc. This also seems unlikely to be the source of your problem.

I would check each plug to see if the problem is limited to certain cylinders and to verify fuel delivery. Sometimes we use a laser thermometer on each header, just past the flange, to more quickly identify the quality of ignition in each cylinder.

Did you fuel up during the last successful drive, before parking the car for dinner? Is it possible that you got some bad gas, or water in the tank? Hold a rag over an empty spark plug hole and try to start the engine, to eject fuel into the rag. Sniff the rag. No, not that deeply. ;) Does the fuel have a strong scent?

Problem solved. I checked the timing and it was advanced at 35 deg BTDC while cranking. I removed the distributor and reset the static timing to about 10 deg. It fired right up and ran beautifully.

I will be changing out all (3) fuel filters as part of the spring tuneup this week.

Thanks for all the input.

Tom, I was about to ask for an update. Glad she's fixed. John

Did the distributor slip? I had that happen once.