So how would you install an external WG on a twin scroll setup?

You need to keep both halves of manifold pressure separate, so that requires 2 pressure bleeds... that would mean either some sort of "split path" WG, or 2 WG's on either side of the manifold collector with the internal WG welded shut right?

 

Assuming you are referring to an aftermarket style manifold/turbo twin scroll, yes, you are correct. Either 1 large wastegate that Y's from the 2 seperate runners, or 2 wastegates.
Dan

 

what about say, an fp black?