I have done a couple of 48" shorties,with stiff handles that handle a 28" draw but they were hop hornbeam. I've never had the oppurtunity to work with osage.However, flipping the tips is a definite! I have never used sinew as a backing yet either (only because I don't have any sinew yet). I backed one the bows with bark and that worked out well though
Wide, flat and very careful tillering will get you there. Those dimensions are the same as my 48" flatbow, but, Like I said, I have no experience with osage so I don't know the difference in the performance between osage and the hop hornbeam.