I had a 69 Chrysler that would do this from time to time. I about pulled my hair out. Finally, just that 1 time I was ready when it happened. jiggling the wires made it start again. Believe it or not, I was loosing the ignition connection through the firewall bulkhead connector. Apparently I was loosing the connection if I hit a bump on the road just right.

I pushed all the wires into the bulkhead connection with a small screw driver from each side of the bulkhead connector and completely solved the issue.

Its a zero dollar fix to give a try. You could also have a lot of corrosion between the connections at the bulkhead connector that would cause the same issue. Gene