I've been following this thread like a mystery novel. I just thought of a wild possibility as "chain jumped" was the first thing to come to my mind like several others. Is it possible the balancer has slipped and TDC with rotor on #1 is just occurring due to luck even though the chain has jumped? Do you know if the TDC mark was correct before the fail to start? I've had 2 small-block Chryslers over the years that the chain died the same way. Ran fine when shut off and then jumped chain.