[dsraven]

assuming the frame is straight and not sagged, first check cab mounts and the space between cab and frame, front and rear of cab, to ensure this is the correct dimension according to the assembly manual, then check/adjust the rad support. you gotta start with the basics first,if the foundation is not right then everything else also will be out of whack a corresponding amount, exaggerated by the distance it is from the source problem.
imagine a surveyor messes up the angles on an intersection.not really noticeable at the intersection but a mile away the road could be 100 ft to one side or the other.
joedoh makes good points. he does this all the time because the frame under his projects is not stock so he needs to line things up "in space" and make the mounts under/between the parts so they fit the curves of the new frame. if he says do this and that then it is pretty bang on that eventually you will need to do this and that eventually. makes sense to check the cab to frame dimensions first though I think. if you think about it like a pile of cadboard boxes stacked together you could figure out which box needs to be moved and in what direction. if you have the old fenders you could try sticking them on for ****es and goggles.