Not sure about the 1300, but the 4500 has another weak point: At the point where the axle
is attached to the frame rail, the frame rail is "split" into two pieces of rectangular tubing that are overlapped and welded together. To overcome this weakness, when my axle
was replaced a large plate was welded to the overlapping tubing joint and then the axle was bolted to that plate. So now I have a bolt on axle.
My 4500 also came to me with "fish plates" reinforcing the bends in the tongue portion of the rails. Also came to me with angle iron tabs and cross members to which 8 body bolts are attached.