Thin wall stainless pipe is going to shrink. The hotter you get it, the more it will shrink. Clamping won't help. Just keep it from getting any hotter than necessary. Weld small segments and stagger them around the seam. Let it get cool before the next segment(s). Not real good for production, but you can get some really crappy results with thinwall if you just weld it out all at one time. Not to mention the carbide precipitation and later rust problems you create.
Steve
DO SOMETHING, EVEN IF IT'S WRONG!