Just as horsenaround said most likely the best place to cut it is behind the cab.I don't think taking out 3" or 4" will be to much of a problem even though the frame is tapered .One other thing to keep in mind is as long ,as the same amount is taken out on both rails of the frame,(say 3") the cuts do not have to be in the same place on each rail,but the amount removed does have to be the same.When I cut a frame I always make sure the frame is square before I start,and scribe a reference lines to work my measurements from.
