[dsraven]

yup, like dwcsr says. grab a tape and a buddy (or a magnet or small clamp to hold the tape) and take some dimensions at the widest part of the frame, since it tapers. then measure the inner and outer parts of the box, like behind the wheel and from fender lip to fender lip. draw it out on a piece of paper like a cross section and do the math on what you you have compared to what you want. you will soon see if you need to tub or not and also what your wheel offsets need to be with your current set up. leave some space for suspension movement, side to side.