I haven't worked with Hackberry, so take this with a big grain of salt. But I don't see why you wouldn't be able to get 60# out of a 66" 1.3 wide bend through the handle bow on most decent white-woods.
Admittedly the width is on the low end, but if you have the bow bending through the handle and you keep it 1.3 wide out past mid limb, you're going to relieve lot of inner limb stress. And 66 is plenty of length. I think its a matter of tillering, if you nail the tiller, my gut tells me 60# is doable with those specs. No sinew needed.
Gabe