I have made a lot of plum bows in the 50 and 60 pound class and have always been impressed with how deep and narrow I can make them without taking any set. Most of the branches I have had to work with were only about 1 1/4 wide max and very high crowned.
Just knowing how wide at the handle for a very heavy war bow would help a lot. I am thinking about 1 3/8 or 35 mm. approx.
Biggest challenge I have had with plum is drying it without it cracking so bad I have to trash it. I am going to try cutting it down to near demensions and then wrapping it in saran wrap. If the bark is removed or the branch is split it will crack to the core within a couple of hours.
I wouldn't do that just cuz of mold, I have been down that rout and had to throw away a sycamore because the mold got actually inside of the wood, and that was with big breathing holes and everything. Then again, sycamore is a white wood that is very susceptible to mold. At least that is what happened to me. For staves that are hard to season without checking or splitting itself apart, ones like the size of the one you described, the best success I have had is to make sure to either split the wood to the pith or to remove the excess belly wood to the pith. Then, season it slowly inside your house in a room that does not have a window open, or an air conditioner in it. Maybe tucked away like in a corner. I have seasoned staves like honeysuckle with no problem this way, and that wood seems to be impossible to season any other way. I think you might of been the guy that actually told me how hard it is to season honeysuckle,... if you were, man were you right,
. With that stuff, you have to decide where your back is right after you cut it and remove the belly to the pith if you want the stave to stay together for more than an hour.