Hi Chris, Your post doesn't state the diameter of the trunk. Having cut several English Walnut trees, most young trees with diameters less than 30 inches tend to have sap wood that is at least 1/2 the diameter. If we use a 24 inch diameter as an example, the dark heart wood would be 12 inches. That would be the widest heartwood plank, but it also has a unusable pith in the center. This would likely leave you less than six inches of dark wood. If you cut the tree to include the root swell, you might squeak out something from the curved wood there. Pistol stocks probably, rifles less likely, unless you have use of blanks with mixed sapwood and heartwood. You can always cut it, check out the amount of heartwood and make your decision about having it sawn.
If you have a potential need for some rally nice english walnut, I would consider trading for lock work. I'll send you some photos
Ron