Hi Rich...I did not have to use any back spacing...a friend gave us 5 original steel rims from his '65 (he wanted to have
wire wheels). The wheels he gave us had 215 75 tires mounted on them...so we were able to test drive and make sure we had clearance for the skirts ....we did... but I understand that this is sometimes a problem be careful!!!
As an interesting side note..when I first bought the car it had 14 inch wire
spoke wheels...they fit just fine over the
disc brakes...I then bought new 14 inch steel wheels ...they did not fit...that was when we were given the 15 inch rims...by the way the 15 inch rims, with the wider tires, do not fit into the original space in the trunk...so I mounted our
spare tire flat (like in a '955)...you could use a narrower spare that would fit...hope that all of this makes sense!!...Ken ?