You say you bought billets ...do they need to be spliced in the handle or did you buy slats(full length sawn boards)?
If you need to splice billets at the handle, you will have to back it with the hickory, at least over the splice(handle area) but you might as well do the whole back.
If slats, you can use either backing. The rawhide will protect the violated back but not add performance. A 1/8" thick hickory backing strip will protect the back but also add quite a bit to the performance.
What I would do is shape and pre-tiller the slat to floor tiller stage(about 4") and glue the hickory backing to it while it is pulled into about 3" of reflex...also referred to as Perry Reflex. You can use Tite-Bond II or III, Urac or other appropriate glues to glue up the backing. I have made a few hickory backed osage bows. They make excellent, hard hitting hunting bows. I would go with 64" to 66" for your 26" draw and you will be able to make weight easily. Pat