I find this interesting. Usually the only blocks that you hear of cracking are the "53's" as stated above. Sounds like its application related to me. How are these engines mounted? Maybe they are mounted in such a way that the block is seeing to much stress or torque.

Most 5.9's esp 12v non encore blocks will hold around 150 psi and crazy cylinder pressures before they give at the cam line.

 

i am unsure what casing numbers these engines were. they were all ppumps so possible they were '53' blocks

 

OK, you dropped in a crate motor.

The 53's crack externally though, don't they? Just above the pan on the left side?

 

Yes usually most 53's crack in the area you described. Passenger side somewhere around or below the freeze plugs.