I'd be curious to know what machine the average automotive machine shop would use to end-mill a block. Likewise, I'd like to know what the fixture to hold it looks like since you'd want to register off the main bores.

With all this talk about the block not being square (which I find hard to believe), why hasn't anybody suggested checking it ?!?! It can be done very easily with the dial indicator and magnetic base using a slightly longer arm to get the indicator out to that block surface. This is a hell of a lot less hassle and way cheaper than having sh!t machined on a whim!