Figuring fit starts with determining the obstacles and gauging how much clearance is available. On the typical Mopar chassis, the first obstacle to the inside is the edge of the leaf spring, while outside we have the fender lip edge to contend with. Both these obstacles can be maneuvered for more clearance if you're determined. Leaf springs can be moved inboard with offset shackles or, more dramatically, with a spring relocation kit. Both of these mods require the spring perches on the axle to be cut off and new ones welded in a new location.
Want more? Here's where mini-tubs or tubs come in, both terms for modifying or replacing the inner wheelwell for more tire clearance. To the outside, the first interference is the fender lip. On most cars the inner fender lip angles straight back from the quarter-panel to join the inner fender in a pinch weld. The edge is usually rough, sharp, and in about the same position towards the tire's sidewall as a loan shark's straight-razor would be to your throat. The fender lip in the critical area above the tire can be "rolled" upward and back towards the quarter, yielding a good half-inch of clearance. For even more clearance, the fenders could be flared, a procedure popular in the '70s but generally frowned upon today.
Next to a trial fit, checking for clearance is best done using a wheel close to what will be run. On this Dart, we had 15x7 cop wheels and 235 60-15 tires all around. We wanted to run the 15x8-inch Wheel Vintiques Cruzer in the rear for a deeper "dished" look, but we weren't sure if it would fit. There are three measurements to be aware of when selecting rims. Obviously, the first is the rated width, 8 inches in this case. The width is the distance from bead to bead inside the rim. More important to fit is the overall outside width of the wheel, which for our 15x8-inch Cruzer is 9 inches. Next, there is the backspacing, or depth from the axle mounting surface to the outside edge of the rim. On the wheels we were considering, the spec was 4 1/2-inch backspacing, or right on the centerline of the 9-inch overall width. Our cop wheels have a 4 1/4-inch backspacing, so the new wheels would be 1/4 inch further in, and 3/4 inch further out, to make up the 1-inch difference in width.