I'm not sure how much difference there is supposed to be between the bore and piston but looking at your dimensions you have 67.75mm for the bore and 67.5mm for the piston. That's a .25mm difference.
.25÷25.4= .0098 difference. The piston will fit that bore according to what you measured. Is...
That thing is great. You've done a fine job building and chronicled your build very nicely. I hope to see some more of your future endeavors even if they are not mini's.
Thank you sir for letting us be a part of your build.