These bolts seem to shear as a by-product of harmonic vibration & flywheel bolts that are not as tight as they should be in this application. [Or just plain loose bolts] Our "A" series engines do not have a harmonic ballancer & i will be investigating the possibility of adapting something for mine.

One of our local blokes has sheared of the bolts a number of times in his B110 Coupe racer & the fitting of a severely modified Falcon [i think] ballancer seems to have overcome the problem.
He was able to re-use the crank too, so get the busted bolts out & have a good look. Your crank might just be OK, & thats gotta be good in the $$$ department.
Chris