You are waiting on a welder, so, are you going to make it a rigid? I would suggest removing the whole back bone, and making a whole new under carriage, along with the back bone, this type of bike has no undercarriage frame wise, they are really reliable motors, but, do not go over board with the front end and such, I did on an exact one just like yours, and because i made a five foot front end for it, it slowly died of blow by, so make it low and stout, so you don't over do the motor for what it can handle, the only thing from my cm400t that exists, is the neck, which is upon my xs650 chopper, along with the five foot front end that i made from stainless rod, not tubing, but solid rod, for strength, and it is totally non magnetic!
here it is!! since the 400 was doing the blow by real bad, I went ahead and built up this frame and hacked off the neck, and just rolled it over to the new bike for the 400 registration, and the plate upon the frame, and it thus became a yamaha!! only I know it though, the insurance does not care, so, neither do I!!! the yamaha had no paper work, so this was why I did this!!