Thanks to all that answered.

The car was back together yesterday afternoon, and ran like a champ.

Taking out the #1 spark plug for the rope trick even led me to the loud and annoying exhaust leak so I could fix it. Somehow the nuts that hold the exhaust manifold to the head were very loose for the #1 cylinder. Bumping them accidentally caused them to rattle and caught my attention. They were that loose. Several of the manifold nuts were loose, but none were as loose as the two on the #1 cylinder, they were about a half of a turn loose, maybe a bit more. All are tightened down to about the right torque now.

The car now runs quietly, like a 740 should, and seems to be much better and smoother. More power when accelerating from a stop too. I think that may indicate that the belt may have slipped a tooth before it failed. The failure was not splitting apart, but a bunch of teeth that were stripped clean off of the belt, so it may well have slipped a tooth or two before enough teeth let go to stop it dead in its tracks.
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1955 Human, Scott; 1991 745 Turbo, Thunderbolt; 1990 745GL, Posideon