How in the world can you drag that weld job out to a few hours? Heat cycles with an industrial oven?
When I welded mine in '98 There was gunk on it that wouldn't let the weld act right. We burned that off with a torch then welded it. 100,000 miles later, it had a small bad spot so I fixed that and added a triangle (6 pack of beer at the muffler shop). It never cracked again.
Got some used stock ceramic coated headers off Ebay around 2000 with a small crack. Welded in a triangle with a MIG. Hasn't cracked yet.