68 cuda tire size

Here's what I did when I had my rims made:

Take off the wheel. Set the axle on a stand that has the axle at ride height. Run a plumb from the spring, a plumb from the rear drum face, and, a plumb from the inside lip of the fender. Take your measurements and you'll know how wide a space you're dealing with, and the correct offset. I recommend doing both sides to ensure that one side isn't a little different.