Just a very short note. I found out while setting up my cowl steering that the corvair pitman shaft has the same treads as a 5/8" rod end. Since that works I'll take an extra tie rod I've got and use it for the shaft running from the box to the pitman arm on the outside of the body. Not sure if it helps anyone,But still a bit of info. that might help.
I don't know for sure but I'd be a bit concerned using a fairly thin walled tubing as the shaft for my steering box. may twist and fatigue with time.
The one I've got has 1/4" wall,Also putting a sleeve over the shaft and then a roll pin. With the treaded part all the way in plus pinned it should hold.Only going to be about 14" to the outside.'27 roadsters are not what you would call "wide"