These are not cracks but factory welded porous spots inside the case. You may see these if you remove you sump and look up inside. Normal stuff and not to worry...unless they are weeping oil.
Dents, burrs, etc. from previous gaskets
Worn-out spots on contact surfaces, especially in bearing lands
Loose studs and stripped threads
Loose flaring or pressing-in of pipes
This is a view looking the the case cavity from rear to front after the main bearing/seal plate has been removed. In the bottom left you can see two press fit oil lines. Check these with solvent or water for leaks. Use epoxy to patch any leaks.
Wear of Camshaft rear bearing (bushing)
Looking through the cam shaft hole to the rear you can see the bronze bushing that the aft end of cam shaft rides in. In the following picture you can see the end of the pressed in bronze bushing about the opening for the rear main bearing/seal plate.
Scores on the Oil Pump drive bushing
When removing the oil pump, inspect the drive gear located under the cover beneath the alternator.
Wear or scores in the holes for the breather on the gearbox cover
When you have the front distributor cover off, inspect the breather that rides on the cam shaft gear. Seen in this picture just poking up a head of my fingers.