With a piece like that, you can just chop away the excess from what is going to be the back, that will help it season. Also it will provide a weak point for any drying cracks to run out to. E.G the cracks radiate out from the centre on the easiest path, so if you have chopped away most of one half the cracks will go that way, leaving the other side good.
That may not make a longbow, but it woll make a nice primitive with the belly looking like a sandwich of sapwood with a filling of heart wood.
E.G. Basically half a log with the back left untouched or with rewduced sapwood. Nice and wide across a flat belly which has sapwood edges and a heart wood centre.
Sort of thing you can rough out with an axe or bandsaw quite quickly. It's the reducing the back in a sensitive manner that takes the time.
A Yew bow of that shape will be quick
Del