The original bellhousing cross member added considerable stiffness to the frame at the rear spring hangers. With our without any mounts it it's a good idea to have a cross member at the front spring rear most hangers.
GMC 400 series didn't have the same cross member and engine mounts as the smaller trucks, instead they used a large U shaped channel iron bolted outside the frame just behind the spring hangers. See attached pictures of this 400 series cross member adapted to a smaller 2 ton GMC. I can't find a picture of one as fit originally in the 400 series GMC.
