I think I may that trick about leaving a bit of slop in the tab. Mine was nice and snug, welds were pretty solid. The tab cracked, and off flew the muffler after catching under the running boards for a bit, I ran it over. I managed to hammer it back into shape, and welded the tab back together. It's stayed together for 7 or 8 races since then, but I think that racerrad8 may have hit on it.
We had 2 other mufflers fall off after their tabs broke. Nowhere near the welds, just cracked the tab. There's obviously some stress there. I was thinking about adding another tab on another header, but now I'm thinking I'll just add some slack to the tab. Thanks racerrad8!
Mike