Very unlikely that your stands are cast iron. Cast iron is tricky to weld, and requires special nickle rods.
If you wanted to get a little more complicated, you could fabricate a removable insert that would slide into the end of that 'upside-down' tube. That way you could remove it, and use the other end as needed.
For a 'one off' solution with your arc welder, it may indeed be easiest to simply cutoff the existing saddle, and weld on a flat one.
1986 300E 5-Speed 240k mi.