It does look like there is some run off on your board, but if you back it then you should be good to go up to about 50# in a 66 ntn bow, but you may go ahead and make it 72" if you have that much. The good thing about it is if you break it then it's not a big hole in your wallet if you start over
go for it and if you break it get another one. I have seen some people say they use a paper grocery sack to back their bows, never tried it but they say it works.
That contest sounds like fun, I was actually toying around with a piece of red oak lumber when I was making my trade bow but decided to use a piece of Osage instead. Only reason I picked Osage was not everyone has access to it, so that made up my mind.