I am surprised at how much a properly fitted piston can rock when you do a test like you did, but yours does seem extreme.
You would think you'd see some wear on the bore at the skirt location though if that was the issue. That being said, I'd pull one piston out at a time, look close, see if you can see any tracks making the noise, or a magical disappearance of the noise when turning. If nothing looks wrong, and the noise is gone when it's just the crank and cam in there, bore gauge and micrometer, check clearances and bore shape, and recommend you do it with a head bolted on from the bottom at this point.
The snap type of noise, in the valley turning slow, makes me wonder. I didn't have that on my flexi-445 a couple years ago, but I had all kinds of odd noises when running, and they all matched yours. It was just too loose and too crooked. FYI on that one, I tried to take a shortcut with a machined block, no deck plate and I knew it was on the loose side, never would have tried it if it wasn't my own engine. Never again LOL