It's lunch time in the far NW of NA, so a quick update on the progress.
I'm a scrounger and a saver, so I end up with lots of bits of this and parts of that, plus metal of many shapes and sizes. And I like to use it as long as it's appropriate for the job and doesn't look or perform like scrap when done. With that in mind, off to the treasure pile I went and returned with a 3X3X1/4" angle and a 1 1/2X3X1/8" tube. The tube I zip cut through two opposite corners to give me a couple of 1 1/2X3" angles with rounded corner profile. Those will fit inside the frame flange to "sandwich" the frame. Its' great time to use up some of those "should be thrown out" zip discs.
The angle got cut length wise to leave 1 5/8" on one web, same width as the tractor frame flange. (ok a smidge more) Then cut into four pieces of an appropriate length. This will all be the underframe.