Machine the flange and use high temp RTV. I hae never had a leak that way but the flange needs to be straight. What most people forget about a turbo application is the fact that there is alot of back pressure in the manifold compared to a N/A app. When I ran my old motor I had 26 psi of back pressure for about 14 psi boost. With the thin layer of RTV and a straight flange the pressure has no gasket or weak area to push out. It just seals.