Diesel engines are exceptionally heavy, with lots of cast iron... the Cummins 5.9L/6.7L (it's the same series of "B" blocks) top the scales at over 1200 Lbs. To answer the direct question- NO, the block will not even crack from a 300BLK round. But cracking the block is not the quickest way to kill modern engines.

Every Cummins since 1998.5 has had some level of electronics... the VP44 engines would be vulnerable in some areas, the Common rail engines would have a LOT more vulnerable parts... hit ONE of several critical electronic part and it turns off immediately.

My 12V would keep going for quite a while, unless you manage to hit the AFC housing (part of the injection pump, and would likely result in the fuel rack going slack... sure it'll die eventually if you take out oil or cooling systems, but it'll last long enough to run down whoever shot it (I'm not endorsing that activity, just saying that don't count on knocking one out that easily)

This is a fun exercise... maybe I should make a armor plate for my AFC housing... hmm.