3 flap or 4 flap or bark grafts are the way to go with walnuts and hickory/pecans. Or, thin caliper seedlings 1/4" or less are easy to whip and tongue.
On walnut you put other species of the same Genus. No to Carya. Juglans on Juglans and Carya on Carya. Speed of growth is a quality you should always consider. Putting pecan on a shagbark hickory many years down the road looks like this:
That’s my good friend Gary Fernald who has selected many far-northern pecan cultivars over his lifetime of traveling up and down the Mississippi River for 30-years… among other fruits/nuts selections.
Dax