PhilG:
Someone in the thread suggested that the ground strap should be replaced, which I intend to do, but I inspected it and found it to be in pretty good shape. Maybe not "like new" shape, but the braid and the ends were in tact with just a couple of individual strands freyed. Is that enough to kill a good ground?
If not done already, remove that engine grounding strap and "renew" the metal-to-metal contact faces at both ends. Significant corrosion between the metal contact faces will greatly restrict the flow of Lucas smoke. See forum member Dave DuBois's web page for a host of things to do to clean up decades of electrical corrosion issues. Good info. A few broken strands will not effect the cable's electrical capacity. Did you try using a jumper cable as a temporary grounding strap between the block and chassis?