wrong spring ?
I've had 4 versaille rear ends over the years. the spring should just be a little longer than the key, so the ends hook over the key. My one ton ford HD dayton brakes were a lot bigger, mabe someone gave you big truck parts. go to a wrecking yard and check out front brakes from mid sized ford cars fron the 70's mabe ealy 80's and see if you can find a spring to match your key. I ve gathered 80's lincoln town car rear discs, they fit on mustang 8.8 with ranger 5 bolt axle shafts , they were cheaper than the explorer 8.8 disc and easier than looking for another 9 in versailles-or monarch 9 disc