Alrighty then, now we got some numbers we can work with. Sounds like your bow has to "shoot around" a pretty normal kind of handle for a self bow, so that is a zero sum. The standards for arrow spine were intially worked out for this kind of bow, from what I have read. If you had an arrow pass cut out of your handle so that the bow was perfectly "center shot" then we could get away with stiffer spines without much problem.
So that means we only have to work with figuring the spine weight for your draw length. The spine weight ranges for arrows are based on measuring between 28 inch supports and a weight hanging exactly in the middle. That means if you cut your 40# spine weight arrows exactly 28" long and mounted 125 grain points then they should fly pretty straight off your bow. But you shoot 24". Generally, for every inch you cut down from the 28" you add about 5# of spine weight to the arrow, for every inch above 28 inches that you leave on the arrow you subtract 5# of spine weight. For now, I would recommend you purchase some arrow shafts from a dealer, get something in the 35-40# spine weight. Cut the shafts to about 29", fit on the field points and nocks on, say, three shafts. Shoot 'em bare shafted at a target a few times. If you are right handed, those arrows should have the nock ends "kicking" off your bow to the left. Shorten and inch and shoot again. When they stop coming off sideways and fly straight, your arrows are the right length for that spine weight shaft.
Since you have the willow shoots, scrape off the bark, sand 'em smooth, and put your points on the fat end, your nocks on the skinny end and do just like I described above for each of the shafts. Once you have them cut to the length that they fly fairly straight, add your fletching to finish them out.