Welcome to the Stovebolt Page! There is a forum called the Welcome Center where you can introduce yourself and your truck.. Does your truck have the original torque-tube drive (enclosed driveshaft)? It is possible that you suffered a rear end failure, but more than likely it's a u-joint... What do you mean by rear output pad and mounting pad? Do you mean one of the rear bellhousing mounts? I'm going to guess you are still learning about these old Chevy trucks, so I will try to guide you to a place with more information.
48-51 Truck Shop Manual This link goes to the page in the shop manual that describes the engine mounts. That entire website is chocked full of old Chevy information. So make yourself acquainted with it. There is also info there about the u-joints, etc.
Could you post a picture of the mounting pad that is missing? On the left of this page are instruction for posting pictures in your post. Since you are new here your posts are monitored and you may have to make so many post before you have permission to post pics.
Good luck.
