Causes of engine block crack?

I would sleeve that, bore to .030 and build what you have. Add a 4" crank, and you could have a nice 416. As far as the magnum question, use oil through lifters, hollow pushrods and a set of PRW rockers. It's that simple. Nothing complicated at all.
Is it really that simple? I have morrel hydraulic roller lifters that are oil through, trickflow heads and Harland sharp rockers for LA blocks