My friend went through a similar problem with MP. It turns out a small run or a lot of them left the foundry with some porous problems in certain places within the blocks. This turned out to be not so bad being that the motor is a very mild REAL street build.
I would never purchase a MP piece EVER just on the principle that they (MP) did not want to own up to the issue. It drove the machinist and builder (later to be revealed after clearance) crazy trying to get an answer or a replacement at the very least. I mean, it's all about customer service right? e$pecially with a brand new block ca$ting right? MP did not want anything to do with it. They said, eat it.
At the end of the day, it's almost worth it to seek and purchase a good old casting from the old Chrysler corporation and build it up from there.