The belly should be parallel to the back. By all means reduce the thicker side so things are even, you'll get twist otherwise.
Other than that, sinew backing an ash stave isn't necessary because the wood is so strong in tension. On such a long stave, sinew will just make the bow sluggish. Save your sinew for short recurves instead.
If your stave has little early wood/if it is very dense, you can definitely make a 100# + bow out of it. There are some people who have made very serious warbows out of Ash.