FOLLOW UP: Thank you all for your insights. It's been over a month since I started this thread, and the pain continues. The only concern at this point is why no spark. I feel I've done everything I can think of. And yes, the two batteries are grounded on the engine block.

Recap: Had no spark in any cylinder except for a very weak spark in #4. Pulled #1 plug and it was black with soot from cranking and stumbling I suppose ; unusual on a new set of Iridium plugs.

I installed a new ignition module and no difference. Pulled the DUI unit and replaced it with a good, standard HEI and got it to run, although badly. It was obviously dropping cylinders.

Checked spark and it's sparking on cylinders 1-3-5-7 (left bank), and only #4 on the right bank.

It has new wires, and there's no mix-up. Cap and rotor look fine.

The ohm meter registers 12.6 Volts at the ignition and the dizzy, and 10 volts when cranking. But that was checked earlier in this long process, so I will check it again for sure.

Any more ideas? I'm heading for the looney ward!