Using the piston stop I',m going to assume it was an adjusatble version that allowed you to keep backing ti out to find the exact TDC. Or was it the version that you rotate both directions, making a mark at each stop and splitting the difrrence ?

Another long shot but easy and cheap to try. Disconnect the vacuum advance and see if it has any effect. excessive advance
(doubtful) can cause misfiring. A cheap try