Sorry, I didn't read all the other reponses. But with good osage one can get a way with quite a bit. These two bows are 58" ntn to draw ~60# @ 28. I think they are inside of 1 1/2" wide at the fades.
You'll want to concentrate the bending more towards the inner limb as you reduce the lenght, and hopefully pick up what weight you'd like. At this point you might be able to heat bend a little more reflex into the bow, if it has not too much set so far from it's original shape. That would help w/ your tiller, if you concentrate the reflex on the outer limbs, and save you some wood, ie. draw weight. We're talking an inch or so here at the string grooves, nothing dramatic.
Your tiller is very close, were it not for the low draw weight you'd be done. As you shorten the limbs, this will naturally put the bending towards the inner limbs. I would not go below 60" ntn, depending upon how much bow you require. And I'd take it in two passes, 1" off each end each time. And be patient, and keep your wood dry, and work it very carefully, and rest it frequently. It's more like a seduction we're about here, rather than the old "breaking a horse to saddle" paradigm where we force our will upon the bow stave. Just listen to it, it wants to be a bow too.