You really need to decide what size tires you are going to run first.
For instance, if you tires at 10 inches wide, and you want the wheel mounting surface centered in the wheel, and the tire exactly 1 inch inside the fender, you then have some basis to calculate axle width.
I don't recall the exact measurements, but if the distance from bed wall to fender is 12 inches, that would mean the wheel mounting surface on each side would need to be 6 inches outside the the bed wall. If the bed is 50 inches wide, your axle would need to be 62 inches.
My numbers are all ballpark, but if you measure your stuff and choose your wheels and tires first, you should be able to determine what axle width will do exactly what you want.
The next challenge will be finding one the rigth width...
