If those are older pistons, Ohio,Badger and others compensated for the increase in compression brought about by the overbore by reducing the pin height....normally I find them to be .015 to .020 less, given the variances in the block deck height this is probably what you have encountered.....interestingly the last set of 307 (TRW) I used with the chamfered head (couldn't find true flats) came out at zero with no decking...the variation in parts can be different from one production run to another I suppose.....Fats
