As others have said, possibilities are
1....Slipped outer balancer ring, check with piston stop
2.....Timing chain slipped
3.....As in the quoted post---reversed pickup wires known also as ROTOR PHASING. Reversing those wires changes the spark event in time in relation to rotor position. [URL]https://documents.holley.com/techlibrary_rotor_phasing.pdf[/URL]