I have no experience with J's, but you would do well to measure the remaining casting at the thinnest spots, over each basenut flange:
Measuring is easy with a common "pincer" caliper that has had the top spring removed, and the adjusting screw reversed. Thus the tips can be opened to go around the flange, closed and carefully set with the screw, and re-opened so that the gap can be measured with feeler gauges.
Searching for the very thinnest spot is what takes the most patience, as all four basenut reliefs must be inspected.
With later cylinders such as ULH's, I have found .070" remaining wall at the thinnest spots to be safe.
It is manditory, however, that the cylinder be bored a full .003" undersize of the piston, and then torqued to a stressplate as if installed for the final hone-fitting.
Once you know your bore, compression height, etc., you might want to try Egge Machine, even if only to determine other machines that used similar pistons.
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