Cool project. John's right, it's like trying to give an estimate over the phone. For body openings it's best to use the doors etc. in place and check the fit visually. For length use a tape & measure & compare from one side to the other. For measuring for "square" measure in an X pattern, going from a bottom corner to the opposite top corner, than do the same from other direction. For window openings it's the same. I've used 3/4" square tubing tack welded in place to cross brace in an X pattern when possible.
