Given my experience with annealing/hardening/tempering steels at my job, I'm not a big believer that any welding done in addition to the factory weld will help. If anything, it can dramatically hurt it. Unless the type of steel is known, and the entire header is heated to the right temperature when it's welded, AND it's properly quenched so the final temper of the steel is known, and even all around, you'll always end up cracking again. With a typical weld, you'll either anneal the welding zone, or make it glass hard. Its just begging to crack again if not done properly. I think a factory replacement header should be left alone, in case you're considering welding up the seams further to make it more crack resistant.
I remember in my Cobra days, the hot ticket shorty headers were JBA's. They had a reputation for cracking at the welds. I bought a new set, and decided to grind out the primary entrances to match the gasket. The metal should have been very hard, and I should have had to use a stone in my die grinder, but the metal was as soft as iron cause the welding process wasnt done properly. They had annealed practically the whole header. I just sold them instead of putting them on my car. I later put out the word as to why these headers were cracking.
I know this isnt really inline with what you're asking. Its just food for thought.
I would personally stick with real GM headers. Quality control with the aftermarket doesnt come close to the QC with factory parts. The parts were designed by real engineers. Not some ex-muffler shop guy.
