Yes, tire/wheel size will determine axle width. Plus frame clearance also. Unless you plan to start reworking the frame rails you really need to be sure the axle width is enough to handle whatever backsapcing the wheels you like will handle, plus clearances.
I'd do the sheetmetal tubbing first. Then once it's done you can get measurements from the existing axle to see what space you have from inner wheelwell edge to outter edge. After that you can measure the hub face to see what you have for outside and backspace on the car.
Once you know the figures you can determine what wheel/tire will work. Most tire companies will give you tread width, but they also have sidewall width or cross section. That will be the true test of what clears, as it's the widest portion of the tire. I like to leave an inch inside, and about 1/2"-1" outside depnding on what side deflection your rear suspension might have. My panhard bar keeps my axle pretty solid, but normal deflection is less than 1/2", so I like 3/4" clearance to the tire for a little room.