The Corvette front spindle center is 5/16 inch behind the ball joint and spindle upright center. That means the center of the wheel is 5/16 inch to the rear of the center of the new crossmember. So the spindle (not the crossmember centerline) will locate on the frame centerline (vertical line you scribed on your framerail). The easiest way to do this is to mark a vertical line on the center of the upright portion of the new front crossmember on each side and then mark another vertical line on the crossmember upright 5/16 inch to the rear of the crossmember centerline. This is your spindle centerline or the center of the wheel when installed.