THe 6.1s manifold is cast alum.,so I kinda doubt it would crack(not saying they can't),The 5.7s are plastic and could be more prone to this,but I've never seen or heard of one cracking.The pcv could be stuck/not working correctly allowing oil to be pulled into the intake under hard acceleration . Whatever it is,you need to do a leak down check or a comp. check,D.