I think you should try for the stiff handle and begin tillering the bow. 66" long with a 4" handle should work fine with a bow 1-3/8" and made of sound black locust. Keep the belly flat, of course. The majority of the bending should occur in the widest parts of the limb with the last 4" or 5" of the tips not bending at all. The bow is a a bit narrow for my taste but you've got to go with what you have. For this bow a goal of 50# at 26" draw is safe, IMO, but if you want 55# at 28" draw you'll need to take it slow and probably get the handle working a little.