Sew ups are well known for not being of any final "size" until mounted. The casing has a large amount of stretch and can often seem smaller in what might be the final size then what is the reality.

Years ago we were clearing out the adjacent to the bike shop house (had been used as satellite storage and work space for years prior) for it's becoming a residential rental again. I came across a couple of orange Clement sew up tires. They were smaller than a 590 ISO tire's bead diameter when lightly inflated. I questioned their size and the boss said they were the usual "700" size. Sure enough I dry mounted them on a pair of rims and they fit (although tight) and inflated to the usual shape. BTW I was also told to not ride them in the rain. I glued them up and had a few weeks of heaven, the nicest tires I can remember riding.

I suggest test mounting. Andy