All of the advice Clapper Zapper gave you is sound. After 25 years of seasoning, your blank should be quite stable, and any slight bow in the grain would be less than you'd often find in a blank with any figure at all. The overall layout of the grain in the blank, particularly at the head and wrist, is much more important, and your pic doesn't show us how suitable it is for a gun stock.
I had some locally grown English Walnut sawn up last fall into 2 5/8" thick slabs. A band-saw mill was used, so the surface is pretty smooth. Some people recommend 3" thick, or even more, but I think that leaves a lot of chips on the floor, and might even get you into wood with less figure. 2" thick is getting about minimum for most guns, and doesn't leave much to work with. But I have a Hamilton Model 27 .22 rifle on my project list that could easily be stocked with a good 1" thick blank. And 2" would be more than enough for a Marlin Model 37 pump .22 with a broken buttstock I recently picked up for cheap.
