If the block was properly honed and mic'd and the crank was cleaned up, polished and mic'd I cannot see a reason for the failures other than shoddy parts or shoddy workmanship. Maybe the previous builders were to lazy to spec everything out for themselves and used the previous builders specifications. If this is the case, and the original builders specs were faulty, then the rest of the rebuilds would follow suit. Amazing what a .010" difference in clearances can make.