If you want to sell the walnut, i'd run add's in the local paper that it's forsale... Unless it's something very spl., it's worth about $2.00 per bd. ft., and i'd sell it ALL to one person. Don't let buyers start picking through it, as you will end up with all the unsaleable culls!
If you want to glue up a few boards for a stock, then cut off 3 boards, about 6" longer than you will need, and take them to a cabinet shop to be planed. Personally, if you haven't done it before, i'd just have the cabinet shop, plane/sand and glue the 3 boards up into a blank for you, using waterproof glue.
If you haven't ever made a stock before, it's a LOT of work, and if you want it to come out right, it's a LOT of work just to inlet the blank to accept the action. I'm not saying you can't do it, but i AM saying it's a LOT more work than you think it is!
I've glued up stock blanks, then sent them in to have them semi-inletted for the gun i wanted to put it on. Maybe that would suit you, but it won't be cheap either.
DM