How wide are you planning to build the bow?
I would probably be going 68" and width of at least 1.5" preferably 1.75"-2".
2" Is definitely wider than you need, but for a first bow a good bit extra wood won't hurt. I think 1.75" is probably close to ideal.
Never shot an elk, but my experience shooting deer leads me to believe it's as much or more about a heavy arrow than it is a heavy bow. First deer kill with grad gear was 43# @ 25" with 500gr wood arrow and I'm sure I didn't get to full draw. Penetrated more than 12". So anyway, I think if you can make a well tillered 50# bow you should be fine.
Lastly, get some good pictures of the board. A backing really may not be necessary, and honestly, and fabric backing or even rawhide will add mass and not any more draw weight so it will slow the bow down. If I was worried I would drop the draw weight a few pounds before i added fabric if the board looked good. At the end of the day it's going to be the bow with minimal limb mass that shoots the best, not the one with the highest draw weight. But that's just one man's opinion, and I admit to never having built a hickory board bow, although I enused many hickory backings.