I should start by saying I haven't built an osage bow, but if I understand correctly, your concern is that the back will have a very high crown.
I think if you're going to make it 68" long, you can probably go pretty narrow and still hit that target weight, say 1.25" or so, so you'd probably be fine doing something like that.
You could also think about decrowning the back and sinewing the bow instead of bamboo ... still not a pure selfbow, but it'd be something different.