Guess you answered my other question from the other forum. Saw that old cast manifold and thought that was what you were talking about. W/ the THS header, you just want to match the port. Take the header, and a magic marker, and color the face of the mounting flange. Then take the gasket and use it as a template to scribe inside the exh. opening w/ a sharp awl or ice pic or something. Now, just match the headers opening w/ the gaskets and you'll get it just about perfect w/ the exh. stack on the engine. You want a smaller bit than the one you linked to. You basically want to open the ends of the passage, and sort of "funnel" the exh. gasses into the header. A 1/8 or 3/16" dia. would be better, and be real careful of going too deep in, 'cuz it's real easy to go right thru the thin tube part of the header! Just grind into the thick, flange part in other words, then blend into the tube...