find the tire that will fit the wheel well, then a rim that will give a normal sidewall bulge and backspace. Bolt axels to the rim (old set cut shorter works great) slide a pipe over the axels to keep straight, let the car down on the tire combo then move or position them to where they fit with good clearance all around at ride height. Then measure to both backs of rim, there's your size to make the housing.
There are no shortcuts unless someone has the identical setup you want (not often)