There are a bunch of trucks with Mustang II fronts welded to the frames and you don't hear about them falling off. Depending on where you cut makes a difference on strength as well as how you cut it. When done right, it should last a life time. Sandwich the cut with same size plates inside and out side the frame rails, long enough to support the weight you plan to have on the frame. I wouldn't put the cut at a suspension mount point. I have seen the plates bolted in and have seen them welded in with spot welds through the center sections of the plates as well as the perimeter.
Joe
