Personally, I would replace the maifold before I cut the headpipe if you are looking to "do it right" from a restoration point of view. You should have no trouble getting close to the same money from a guy who isnt restoring a car, just wants the hi po manifolds. The way to get it to fit now with that manifold, is to cut about 1" below the flange of the new headpipe, install the entire system, expand the section of pipe on the flange slightly, then bolt it on the manifold with the gasket in place. Then you stick the cut end of the head pipe in the "puffed up" area of the flange, and tack weld it. Then unbolt the flange, lower it a little, and weld it solid. Then it's on the same angle as the manifold side, and isnt twisting the gasket area causing leaks every few months.