Have you tried using a laser level to shoot a line down the center of the lenght of the bike. this should be helpful
Jun 18, 2008 Rating
Centering the tire by: Anonymous
First of all the tire MUST be centered in the frame. If the tire is offset to fit the drive you must re-center the tire and use an offset sprocket to for the new width. Obviously the fender should be centered over the tire and 'spaced' in the frame to fit. Use spacers or bungs as needed to securely fit the fender to the frame.
https://www.custom-choppers-guide.com/chopper-frame-plans.html
Rick