Block out of square

Being a machinist I was only halfway joking. I know that I could sand it down with progressively finer sand paper then use a very large flat file to get it within .005" of where it should be. Since the OP already has a proven method on how to check for out of square he should be able to know when it's good to go. But then again, I'm a machinist, and I've had to do even more difficult things with fewer options. the flat surface of the belt sander would make it pretty easy to maintain a flat surface but you have to know what you're doing????
TRUE STORY------I bought a Chevy Vega back in the70s, it had a bad head gasket. Engine wouldn't run at all. The old Vegas had an inline aluminum 4 cylinder that had what are called "floating cylinders". The floating cylinder had a tendency to shift and shrink causing head gasket leaks. I pulled the heads, put a straight edge across the tops of the cylinders and found that the two center cylinders has sunk about .060" down into the block. I took a 2 X 4 and some coarse sand paper and started sanding down all the high spots on the top of the block. Of course I filled the cylinders with rags first to keep them clean. Either way, after a few hours of sanding I managed to get it pretty darn flat using my 18 inch straight edge and feeler gauges to verify the flatness. I bought a new head gasket, installed the old head, started the engine and drove the car for 5 years with no issues until I sold it. I told the buyer what I had done and he said, "No problem, at least if I blow another head gasket I'll know what to do." Treblig