Ahhh, the 'weak link' of the A series. I too had this problem with the big missed gear over-rev and I agree that it is an harmonic problem as mine threw off the flywheel AND damaged the front pulley. Having said that, I feel the Datsun flywheel bolts are a joke - they locate in the flywheel like a 'dick in a bucket' so to speak. Therefore you are relying on clamping force only and once you get some BIG inertia and some vibration you have to expect some movement. I have had my crank machined to accept 7/16 UNF aircraft bolts - two of which act as dowels(my flywheel is machined to finite tolerance for these two) and clearence for the others. Hardened and ground washers complete the picture. My theory is that if it comes off again I want half a crank attached to it Doesn't mean cant happen, just trying to eliminate some of the shortcomings.
Cheers