Rear Frame Width

First mount the tires on the rims and measure the installed width on the rims.Add at least 1.5 inches on either side of the tire for clearance and multiply by 2.Measure the distance from one inner rear fender wheel well to the other inner fender wheel well and subtract the number of the tire width you came up with. A example would be this,mt hoosiers are 16.5 on the rims,so you add on 3 inches to each tire making it a 20 inch ball park number by 2=40 inches.If you add in your 28 inch frame rails,your number is 68 and the inner fender distance can not be less than that,or you go to a small frame rail,26 inches.You try to use the widest frame rail as possible for handling,and clearance to work width,Mrmopartech