Not sure how far you've gotten here but, how did you determine there was an overbore of.020"-.030" with no measuring tools?
If you just looked down the bore holes and saw pistons with a big gap around the top crown/land... then you really don't know anything about it yet. Many pistons are even made that way on purpose!
I have not "determined yet" I may not have been clear in my writing this is what I suspect. I was reading 66.3 mm bore with a 20 dollar dial caliper I don't trust and tomorrow I am going to take my quality caliper to take some measurements. But a .5 mm overbore would be about I mentioned .020 or .030 overbore because they are common and again what I suspect. But my writing may not have been clear lol. .5 mm bore difference can be noticeable though. So far the reading with the crappy tools is pointing towards what I suspect. If I am wrong, that that would be great news
. Of course I do have two bent rods that one was obvious unfortunately.
Of course I will update with more accurate measurements. I am hoping I am wrong.