it's hard to find 45-50# spine 5/16" shafts of any wood genus these dayze.
mike @ maddog archery did a
topic post about using 5/16" woodcrafter birch dowels to make cheap $1 arrows. his resulting 29" arrows spine in around 40#.
i bought a hundred of these dowels ($16 + shipping!) and can vouch for the spine rating as well. i've made up six 29" arrows and they all fly well outta my 50-55# longbows.
so far, i've just cut the shafts to length and tapered the ends for nocks and points. the birch is extremely straight grained and the pencil taper tools shaves the wood beautifully in a long micro wood ribbon (much better than poc and other arrow woods). i'm using 70-100 grain field points for a final weight range of between 480-515 grains. i haven't yet experimented with adding tail weight (via an internal nail, using the braveheart/3rivers nail footing jig) but no question that will stiffen up the spine to perhaps 45-50 ... need to trial that out.
personally, i think too much weight is put on shaft/arrow spine numbers, at least from my perspective. even with a close cut arrow window, i have no issues with weak spined arrows if i do my job right, which is to aim the arrow and not the bow. as always, ymmv!