I did this with two 80s mountain bikes. I cut the seat- and chain-stays off the front frame and the top and down tubes off the rear frame, leaving me with a front and a rear triangle. I used 1.25 inch, 0.9mm wall-thickness 4130 from Dillsburg Aero Works to create a boom tube and marathon lateral tube. The boom tube is actually two parallel tubes, for added rigidity.
It's worked out very well -- I've done a couple hundred miles of road riding on it, including ~200 of loaded touring.
FYI / HTH.
Daniel
(jigged and waiting for TIG welding)
(built-up)
(close-up of boom tubes)
(with a full touring load)