Model the parts so they fit perfectly together (as they would if welded) and run it with bonded contact.
Best is to change the bodies so they are in contact. Split faces as necessary so that you can bond edge-to-edge. Then analyze the resulting nodal forces on the bonded edges to determine the size of weld needed.
You can simulate it using beam elements. For that gap location, you can also adjust the gap distance in "joints" definition.
Hope it helps.