As a caution, I read where one racer at BUB a few weeks ago was measured as too big with a 738cc bike in the 750 class. He must have had a lot of wear in that cylinder. I guess if I was ordering custom pistons I might go a little smaller as they seem to have measured his bore, not the piston size. It may not be that big a deal, but if you are doing custom, why risk it.
While we are on this subject, how are measurements done at the ECTA? If you have to tear down is it after a run, or at the end of the meet? My pushrod bike isn't bad, but with a desmo it can be an all day affair.