Weld the upper ball joint in ?

If the threads are in good shape and the ball joint can be tightened good tacks welds are probably overkill. They'll be rust-stuck real good in a few months anyhow. If the threads are stripped, which is a common result of somebody pressing out the ball joints, tack welds are the standard fix.

The upper ball joint doesn't carry weight, so no need to go nuts, a small tack should be plenty.