Cat. Converter on 4 cyl Contour (Mystique)

I want to replace the cat converter on my 2.0, 4 cylinder 95 Mystique with a universal after market, $60 versus Ford's $900 rip off. Does it have to be mounted close to the manifold as in the original, or can it be welded in downstream on the exhaust? Is it possible to cut open the original cat., and simply replace the ceramic core, and weld it shut? Thanks