I don't know the exact sizes, but they're much smaller than earlier ones, so if you're putting a later flatty into an earlier car, you'll likely need a hose bushing on the head. Alternatively, I'm told Flathead Jack has t-stat housings with the larger, earlier sizes...
i just measured a engine that came out of a `50 ford , the upper was 1-1/4" and the bottom measured 1-11/16" ......that makes no sense to me
There are plenty of others who subscribe to different theories than I do, but, I believe that the later flattys had smaller outlet ports to keep relative pressure higher within the block to help eliminate hot spots and localized boiling.... Just my opinion...