Well, I wish you luck. The definitive book on the construction and history of crossbows is Payne-Gallway's old book:
http://www.amazon.com/Crossbow-Military-Sporting-History-Construction/dp/1602390...I got a copy from the library.
There are several problems with making a decent (that is, powerful) crossbow prod from wood.
First, you need to know the essentials of bow-making; how to choose the wood, what sort of grain structure to look for, how to tiller the bow into a shape that will work properly.
Because the bow is so short, everything is rather critical.
Heavy bows are normally "braced" by means of a device called a false string that is clamped onto the bow arms. This is not practical with wood; the bow can be damaged.
Note that Europeans abandoned wood as a material for prods very early in the history of these weapons. The best of them were reinforced with sinew, and probably only drew a bit over 100 pounds.
A steel prod in the 125 pound range will be lighter, more durable, and shoot better than a wood one.
There used to be a page up on the net of a fellow that makes prods from hickory; but the design was pretty primitive.