were this block is crack this is a easy fix you can use a heli-coil but if you do that i would braze the cracks an then use a heli-coil .why would i do this ? to keep the heli-coil from moveing when you draw down the bolt this will keep it from cracking more .if you do a cold fix use a keylock or e-zlok this is solid thread repair a can not pull as much on the cracks. do not use a insert with locking keys when you drive the keys you put more stress on the cracks and make them bigger for this reapair i like the e-zlok or keylocking inserts for this reapair because it is soild insert and will not work on the crack i have use this reapair on heads an blocks and .heli-coils are good for strip thread only or soft aluminum .