Murray bought a hydraulic tube expander for doing these exhaust systems. That device simplifies the process quite a lot. The one pipe slips inside the other so the pipes naturally tend to line up and stay put for tacking.
It took a bit of practice to get the fits just right, but he seems to have it now. When he first got it he was using too much pressure and creating very loose fits, harder to weld and didn't work as well for lineup. With the pipes fitting properly it is possible to do very small tig tacks that dissappear when the main weld goes on.
To answer Mikeymetal's question from way back (sorry, I missed it earlier) , no I haven't made a set of stainless headers for him yet.
Everlast Imig 200
Everlast Power Ultra 205
P&H 400 amp A.C.
Miller 230 amp with Onan power