Question is answered: 273 Engine Block Decking

I don't know how much you can check when all the parts are at the machine shop. If it were me, I'd have checked each rod for center to center length. Now if you picked the longest rod, you could match it with the tallest piston to simulate worst case scenario. I'd temporarily bolt up the piston rod assembly in #1,#2,#7,& #8 holes and measure the deck height. From the measurements you can infer the out of plane difference for each head mounting surface of the block, ASSUMING each crank throw strokes are consistent and spot-on.
As for pistons hitting valves, it is much more complex and there are many other factors to consider like cam specs cam timing, etc. You are really concerned with like 20 degrees before but mostly after TDC. This is where many guys use a ball of clay to take an impression representing the physical valve to piston clearance inside the combustion chamber at the minimum clearance position. It is relatively quick and very visual. But it entails a lot of prep work, mocking EVERYTHING up, making an impression and depending on the results, tearing it back down to correct a problem, or reassembling to try it on another cylinder, etc. This can be done ad nauseum until you're satisfied with the clearances.
This is a lot of work to expect from a machine shop, that's why I do my own assembly and some of the machine work, because I can only trust my work, and that's the only kind of work like that that I can afford.