Is overspray on cylinder head ok?

They look like adjustable 273 rockers. Perfectly fine to run on a 340.
They could have at least pressed a piece of aluminum foil around the valve train. That alone would have kept most of the overspray out of the engine.
If you dont want to pull the engine apart, here is what I would do. Put it on an engine stand and turn it upside down. Spray lacquer thinner on the overspay and use a scrub brush on it. Flush it with more lacquer thinner and repeat until clean.
Put a bore scope in the cylinders. At least then you could see if its got a fresh bore.