callahan did an exact replica, and didn't really tiller it, much more compared the taper of his bow to the taper on the original one, in order to archieve an as correct-possible replica.
I suggest you make yours with similar dimensions, but like 66" long, wich suits better to your length, and is a little safer.
I wouldn't want to mess too much with the dimensions. 1 1/2" - 2" wide limbs are usually okay for whitewoods, if you make it 5% shorter, you might want to make it 5% wider, wich is a few strokes with your rasp. Nothing to worry about.
The holmegaards were mostly elm, the 2nd. favorite wood of the nativve europeans. The first was yew, ofcourse.
Nick