I think it's the Toyotsports Ebay manifolds you're on about? The CT9 ones are pants but the TD04 ones are meant to be okay according to a few members. Mild steel would have less chance of cracking so I'd go for that, but regardless of what type of steel, I'd get the manifold braced to support the weight of the turbo Only thing I can comment on external wastegate is that I think externally gated kits hold boost better? And the only way to adjust boost pressure is to get a softer/stiffer wastegate spring or use an EBC