usually the crack propogates down the side of the weld on the heat effected zone..
weld repair can be done yes, easier than on cast manifolds.
400bhp on sensible tune and control of egt's would make for reliable setup despite the power.
my new setup is gt35, external gate, ceramic coated manifold & downpipe, heat jacketed turbo hotside.. on the dyno yesterday on power runs, not more than 730'C egt's seen. 950'C plus I have seen before on the PD manifold without adverse effect. keep the egt's within the material spec and all should be well. My figures were on pump fuel without my water/meth running also.
