Hi jturner,
I'm no expert but I did get to see some working recurves, before. From what I understand, 64 inches is almost a max amount for osage recurves. More than that and your not maximizing energy storage. So you are good to go there. If shorter by a bit, I think you are still ok but if much shorter maybe just sinew back? The bowyers bible 2 on recurves claims that working recurves are easier for the beginner. I'm no expert though. 1.5" Is plenty for Osage from what I've seen as well.
Maybe more will chime in on this as I don't have much Osage experience. I'm just passing along what I've learned from others and reading.
Good Luck!