Ye Welding Project is completed! No one died! We went to Pies and Pints for dinner! Life is good.
holeinthedonut, Goddess of Flame, did all the actual joining today; I held up the fucking-around-with-random-hardware end of things. I am exceedingly pleased that I thought of adding threaded posts to bolt the welded part to the wooden part, bought the right die, and did half the threading - and freshly cut threads are a beautiful thing. It took a bit of engineering to get it together, but it all Just Worked, and it's done.