The big block single bolt ( for mounting cam gear ) have been known to not hold well enough, especially on modified engines. The gear could have spun to some extent on the cam( sheared the woodruff key ). If you looked at the timing dots on the cam and crank, it would look like it was ok, but the cam would be in the wrong position. It could have seen extra stress when you were timing the engine. A backfire would be hard on it. If you have the three bolt cam gear, disregard this.