The 73 up have a longer wheel base(4 1/2" I beleive). The later crossmember can be bolted under the 69 frame complete to give you the disc brakes. As far as the rear coil set up, it was really a good design, & the later rear diffs can be swapped in with a little welding.......trannys are also swapable. You can swap the body to a later frame, but you will be shortening the frame, & setting up cab mounts,& nodoubt redrilling holes to mount the box. I have mid 70's 1/2 ton parts under my longhorn,but still have the same chassis. good luck.....AL
