Best way to do it is have all your parts laying there. Tires and wheels first. Mount em to your hubs and Bridgett spindles to fit your hubs, possibly brakes, whatever you wanna have. Then measure everything's widths, subtract that from your overall width, then whatever number you come up with is your beam width. From seeing how much beam width you have to work with, and how wide your frame is and where you mount it, makes you decide how much of a drop you need and the angles you'll have to cut to do so
