I'd think 66" to 68" would be better for black walnut. Pyramid would be fine or semi-pyramid with parallel limbs out about 6" before tapering to the tips. Keep the tips wide until first brace so you can see how the string tracks. If the string is off one side or the other you can remove wood from the side of the tips to get the string more centered. Same with the handle, leave it unshaped and as with the tips you can remove wood to get the string to line up. After first brace you can reduce the tips to 3/8" or slightly less.