I would send the block out and have it "dipped" rather than blasted. When I say "dipped" I mean the same process for dipping bodies. This gets every bit of crud, rust etc. from every nook and cranny whereas the blaster can't possibly get into the water jackets. The dipping won't harm machined surfaces but by the same token, won't peen any casting flash either. I have sandblasted my own blocks a couple times. I made wood plates to cover machined surfaces and plugs for the lifter bores. What a PITA. Dipping would be the way to go. On a recent episode of Nick's Garage he sent out a Charger and a whole sh!tload of parts including a block.to the dipper. He was quite impressed with result.