Greg & The List,
With experience from a half dozen Harry Quinn frames from late 1973 to 1976, most were 27.2 for the seat post, seat pin to be country correct. One was smaller, probably by design, it took a 27.0. The frames that had "fastback" seat stays often needed reaming to fit a 27.2 well, 27.0 was tried but not right. I suspect heat distortion of the seat tube seat stay joint. Do keep in mind that not all 27.2 seat posts are actually as marked... Campagnolo is generally correct but the the "superlight" posts were often undersized.
An aside, do check the parallelism of the bottom bracket faces! The examples I have seen with cut out shells often needed facing...
John Jorgensen
Palos Verdes Ca