The boo should be about 1/8" thick at the crown at the handle and, after shaping the boo, a slight taper, if any, to the tips. I always pre-tillered the osage belly so at glue up you'll have a nice even bend. Two to 4" of reflex should be plenty. I never measured before or after so I don't know how thick...I just took wood off until I got the tiller and weight I wanted. TBII or TBIII will work fine for osage/boo and is cheap and easy.
When you prepare the boo, start by sanding the belly side flat, draw and cut out the shape you want and then thin and taper it before glue up.
You should have no problem achieving 55#@26". With the width and length you have chosen. The bow should last you a lifetime. Take it slow and easy and post lots of pics. Pat