On every Holset i've ever run we just oval out the holes in the turbine housing. Grab a t4 gasket, line the ports up with the turbine housing, and oval out the mounting holes till they line up. You may have to touch up and add some clearance where the nut sits on the backside of the flange of the turbine housing as well, its usually been tight in my experience but never needed. (may be needed in your app im not sure)