Twisting Tubes

Sure, weld it. But "brace" it too, as long as you're at it. The reason they twist under hard work is not that the tubes aren't attached well enough in the first place. They are just not structuarlly beefy enough to hold up to big power. The flexing induced under load eventually makes it start to come apart - the "twist."

I'd hammer out four foot-long strips of 1/8 or 3/16 flat bar to fit across the tubes and halfway up the center, stitch them on a little at a time to avoid warpage, working around top-to-bottom, left-to-right.

Then the housing won't be the weak link, see how long the bearings hold up...