look up a local weldery & machine shop. that downpipe looks to be in good shape from the little shown in the pics...409 SS material looks good.
why not have them MIG weld the cut? by definition, MIG uses wire as filler and does not have to heat the part all the way thru as in TIG.
a little surface clean up and it's ready to go. i would not toss that downpipe just because of a little slit from cutting off the cat's collar.