12:1 afr for a track car? that personally would have made me nervous, cracked ring land screams det as well, probably fine for a few minutes at full noise, but with track temps and heatsoak it might not have been enough leg room.
Could have been something else, rev limiter action or who knows but if you replace the motor it I'd definitely drop it down for the track, be nice to get a temp gauge and see what sort of temps it is seeing but sometimes it is just easier to guess and err on the safe side.