I added I believe, the long one. It seems to be the same length as the Tri-glide used by Redwood. I have an 8' bed and with the original pin box, I couldn't open my tailgate all the way. Now it drops down all the way with about 4" to spare. That was a nice surprise. On my 36RE, I don't think any thing shorter would have cleared the front cap.
2013 36RE, 2008 GMC 3500CC DRW