I was hoping for more thread engagement than just the threaded bung, hence my desire to tap it. Since I already have the 1.5" .120 wall tube, I've come up with a new plan. It will involve Johnny Joints with 1.25" shanks. I'm going to get some 1.75" .120 wall tube, sleeve over the 1.5", insert the tube adapter in the end of the 1.75" that will protrude out from the 1.5" just enough for the adapter. That'll give me a thicker wall on the arms, make it easy with the use of the adapter, and the shank that threads in past the adapter will have a snug fit inside the 1.5" .120 wall tube. Sounds good in my head. Am I missing anything?
ps: I like the RE Superflex joints, but I don't care for the way they make them adjustable with the threaded rod piece you can weld on to the end of the tube. I'd rather just have a 1.25" shank directly off of the joint, preferably forged that way from the beginning.