Cracked b20 exhaust manifolds are very common. I have 2 in my garage with visible hairline cracks at the forward section of the "Y". One of these occurred after swapping it to a new engine.

I am goin to try this one at least one of these techniques:
http://www.muggyweld.com/silvercast.html
Or maybe even...
http://www.muggyweld.com/cummins.html

I favor the silver method. Cast iron has to be cooled slowly, or it twists and cracks due to internal forces. After all these years I had hoped that they woudl have worked out those internal forces, the way t6hat seasoned diesel engine block do, but perhaps they get brittle with age.

All headers I've heard are slightly louder through the firewall than stock.