I've seen it done. Basically, you cut the manifold into 2 parts, with one side keeping the heat riser (most I've seen keep the riser on the rear part). You would then weld a plate over the open ends to seal them, and on the part that doesn't have a exhaust pipe flange, you would cut an opening and add one. I've seen nicely done ones, you would almost swaer they were factory made, but most I've seen were rather sloppy.
