Measure from the back of the rim to the mounting surface that contacts the rear drum. That's the back spacing. Google wheel backspacing and you'll get plenty of info.

How wide the tire and wheel can be depends of the space between the fender and the inner fender well and how much clearance that you consider adequate.

One thing that I found out is a large diameter tire might not be able to be removed easily, if the fender opening is much smaller than the tire diameter.

On my car, I was told that 10" wide wheels with a 4.5 inch back space would work, but when I tried to mount the 295/45/20 (30.5" diameter) tires on the car, they would not go on, without disconnecting the lower coil-over mounting bolt, removing the brake flex lines and the driveshaft, to drop the rear end really low. If you have really wide tires (like 18-22"), and a fender opening that's smaller than the tire diameter, it may be the only way to get the tires on and off. Tires like that require a narrowed rear end and a modified frame, to keep the tires under the fender.

I had 1-1/4" clearance between the tire and inner fender well, but 1/8" or less between the fender and the outer face of the tire. I was able to slot the mounting holes for my rear fenders and move them out about 3/8" to get the clearance I needed on the outer face of the tire.