Hmmm... haven't run into that problem before, Bill.
Consider aligning the outer upper surface of the tube where it connects with the upper surface of the 'dome' on the valley pan and using an oxyacetylene torch to braze the sections together with a 1/16 inch bronze filler rod. Brazing with appropriate heating of the valley pan steel and filler tube and adding the bronze filler rod will produce a joint that appears to have been soldered instead of appearing like a conventional welding type bead.
Eutectic Corp. offers some brazing type rods that have nickle added to the usual bronze-only filler. The tensile strength of joints made with this type of specialty filler rod is equivalent to having done a fusion weld with a mild steel filler rod!
I've also seen silver solder used for accomplishing the type of attachment I've described above. Silver solder (like what a jeweler might use) has a greater tensile strength than the bronze filler rod I first suggested. Again, like the bronze filler first mentioned, silver solder will flow to give a nice joint appearance akin to soldering copper plumbing, but stronger!
Hope this helps.
NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York