The rim offset will determine the actual rear end width. If you are going to put everything UNDER the fenders.........You need the entire width to be 63-64 inches. That's the tires mounted on the rims, the wheels/ tires bolted to the rear axle assembly.
To figure this out, you MUST have the tires and rims already in your possession/mounted. Then put them standing like they are bolted to the rear axle and move them in/out until they are 63-63 inches apart overall and THEN measure the distance between the backside of the rims. This is the rear end width you need to put them under a Deuce.