iv seen external gates on standard manifolds, on rb30s mostly. Where the flange to the turbo from the manifold is, where it all goes into one, you draw out your hole then blow it out with i think a plasma or a very high temp oxy (someone correct me) then u get a piece of pipe so you can clamp your wastegate onto it and weld it where u blew ur hole in the manifold, i think you use a tig welder or a plasma welder.