Used a scrap piece of plywood with an appropriately sized hole to hold the tube and bellcrank square and at teh correct relative position while soldering. For soldering, I just put a loop of solder around the junction and heated the whole shebang up. The wood block caught on fire once or twice, but otherwise served its job well.

Here are all the bellcranks soldered to the tubes and nested together.

Fabricated some pushrods from some 3/16 steel rod. For the real ones, I'll use better steel than this hardware store mild steel. Threaded them with a die 10-32 UNF.

Realized I made the rods an inch too long. They need to be 4" long, not 5". Easily rectified.

It works! Here is a short video showing the action. Now to get the real parts machined by a machine shop.