another overheating problem

The issue was probably the head. I'd take a chance on not checking the block, but of course, that is just working the probabilities that tend to have the head warp before the block. YMMV

If you had a known flat hard block of something (like some granite counter top), that was long enough to span 2-3 cylinders, you could use that hold some fine wet-or-dry sandpaper flat and run it over the block surface and just look at the pattern to look for low spots. I've never heard of using glass; I think it flexes some so how good could that be?

Can you take something rigid to the machine shop and check it against the machinist's flat so you know if you have something good? I have an older aluminum level (now aged and stable), 2' long that I had checked against a known straight edge use to true up ways on lathes; I now know that level is within .001" of flat all across so I have a good standard to use at home.

If you go to slantsix.org, there is an excellent article on the studs, etc., for the manifolds.