hope this helps.
Drying wood naturally is a very risky process. I've used a lot of backyard wood for carving before, and it's always a bit of a gamble. A piece that is long enough for a neck is probably going to warp significantly throughout the drying process. If not, you might get to the very end of your project only to have it crack after you've glued everything together.
I'm not saying "don't try it", but just be aware that it might not turn out the way you wanted it to. On the other hand, it might come out really well, which would be awesome!