Dillsburg is cheaper, but they prefer that you buy in full lengths, which are 17 to 24 feet.

I've heard there are mig'd recumbents out there, MIG is never used on high-end DF bikes. There are recumbent frame forums out there, I recently saw references to them and forget where. I have no idea about wall thickness, but for a very long tube you want to go to larger diameters. .049 seems like it would be pretty safe, but for a big tube you may want to go to .035, I just don't know.

As far as head tubes go, I'm not sure that the general 4130 suppliers will sell you the right stuff because on a welded frame, you want thicker walls. But the ID is the most important dimension. Might be better off going to Nova or Henry James for that. You also need access to a head tube reamer and facing tool. You can use either 1 3/8 or 1 1/4 with threadless, although 1 3/8 is more common.

You can certainly TIG the BB to 4130, I have no idea about mig or fillers. I would think that your biggest problem may be shell distortion. The TIG guys use a heat sink to keep distortion under control.