OK,
So here is what I did
1 - Shape my small transducer using Joe's method, real easy and didn't change much but did make it significantly more consistent
2 - Did a drill bit test between 7 and 8 at the center of the freeze plug. Got .215, so using bore spacing and bore calcs that got me to a calculation of .335 for both, at that exact point, or about .1675 of cylinder wall
3 - So measured those points with the sonic checker. I got something odd at the iron setting, too thick
4 - So I went back to the deck, and adjusted the velocity to calibrate for the measured cylinder spacing at the deck - .560. It took me back to the same adjustment as yesterday, velocity of 4675
5 - I went back down to my point parallel to the freeze plug, and got repeatable .165-.166. It's slightly thinner than expected, but pretty darn close to the drill bit test and knowing I didn't jam in the drill bit, there was slightly more space than the drill size.
6 - I then measured all the cylinder spacing at the deck and it came up pretty stable
Something to note, #7 cylinder walls clearly get thinner as you approach the deck, at the feeeze plug, the .2150 drill bit was tight and would not slide lower, however it loosened up as it went higher. This block is pretty clean, it wasn't a lot, 1 drill size, but it was noticeable
7 - So I measured all the thrust sides at about 1.500 down and got a range from .118 to .134, all other dimensions thicker, and as I go deeper it's thicker
8 - I then did a final check against the china wall for sort of a "post-event calibration" and it came up pretty close, not perfect, but it's not machined and it has a good coat of paint on it, so not different enough to concern me.
In the end, Joe's shaping helped, Brent's advice off line to "always calibrate to the deck" was correct, and Heo's quick check gave me a bit of peace of mind.
I will post thoughts in the engine forum on "what next", and later today, likely repeat the process on a standard bore 428 to see if it repeats