[dsraven]

the best thing you could do is actually do a hands on measurement on the truck and a quick sketch as you go. ensure to leave room for some tire flex and suspension movement-side to side- as you go around corners, do burnouts etc

-box width, as if no fenders were bolted on, outside to outside at the wheels, include any bumps or contours at it's widest point where the tires will be
-dimension from lip inside rear fender to the box wall-both sides at the narrowest point
-dimension from wheel mounting surface to box wall-both sides
-check to ensure the truck is the same from one side to the other
-dimension of total tire width at the widest spot
-dimension from wheel mounting surface of rim to the outside of the tire on the back side. lay the tire/wheel down on it's back side, on a flat hard surface, and then measure from the wheel mounting surface to the floor
-dimension from wheel mounting surface of rim to the outside of the tire on the front side. same as above but with the wheel now sitting on it's front side

insert all the dimensions into your sketch and do the math. easy and accurate for YOUR truck with YOUR wheels and YOUR fenders. not everything is the same from truck to truck on these things. don't forget you may need some longer wheel studs if using spacers.now would be a great time to change the wheel stud pattern too, if you haven't done that yet.