Two possibilities: 43-yr-old ignition switches are known to electrically fail. Try a known-good one from another member (yes- some people do lend parts for trouble-shooting!)
Second choice- try a second coil. Can-of-oil coils are known to fail by burning the secondary windings inside where one cannot see them. When the can cools down (which may only take a minute or so), the burned wire ends may re-connect- until things get hot and the ends expand away and break the circuit again. For test purposes, ANY coil will work temporarily. If this turns out to be the problem, I suggest using a transformer-style E-coil in which the windings are exposed. They have been found to be far more insensitive to overheating.
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