Try Motorvation Engineering in Sibley, Iowa. I just happen to own one of their sidecars and it's mounted on my Centennial Triumph Bonneville but they most likely will sell you what you require and they have the knowledge and expertise to assist you. Nice folks. In fact I drove out there from here (Michigan) and had them mount my chair for me.
If I was building my own frame, I'd be TIG welding the joints, not MIG and especially not with the ancient one you have... Just my view and nothing more.
You might also want to join the United States Sidecar Association forum, it's a good site and lots of folks on there will have leads for everything you need as well.
