If a manifold is not built right, here is what can happen.
There were two flaws which led to this outcome.
Metal too thin 1/8"
Ground down welds on metal that was too thin.
To rememdy this, I cut out the 1/8" and replaced it with 1/4". I then angled the cuts so I good penetration so I could still grind the welds down.
What happens when manifolds fail is the flexing of the aluminum. Aluminum doesn't like to flex, and will eventually fatigue and crack. Your filler should also be an alloy that works better for flexing. I don't think 6061 is a good alloy for anything that will have a constant pressure change. It is very rigid, but if you think there might be any flexing at all, 5052 should be your choice, along with a filler that compliments the 5052. I could be wrong in my thinking, but I have also been told similar by others.
Also, Tun_x....we are going to build you one for 1/4 of that price anyway.
Although for $990, that isn't a bad price, that thing has custom runners and everything, with a VERY VERY clean interior. The craftsmanship looks top notch.