DSM manifolds were turd. Cutting a flange off a cast manifold and trying to weld it to a mild steel stock part is asking for trouble. Even if you properly pre-heat the piece, use the proper nickel rod to weld it there is a very significant chance it will still crack. The nickel rods alone will cost the weldor $20
Now what will you do if/when it does crack? you're stuck with a custom downpipe and you're going to need a new manifold that's exactly like the old one, or make a new downpipe.
I'm not trying to poo poo your idea, just point out that I've seen this sort of thing tried many times. In hondas people used to buy an HF manifold and weld a T3 flange on it. When it cracked they'd bring it to me, I'd excavate the crack, pre-heat, re-weld and $100 later they'd leave for another week until it cracked someplace else. It got so bad that I started making logs in the exact dimensions of an HF part. When they got tired of taking their HF manifold off they'd buy the log. $50 for the manifold, $20 for a t3 flange, $100 at Joe's welding. $100 getting the cracks re-welded with me. or $350 for a stainless steel log...
I just hate seeing the same mistakes made over and over by every generation of car enthusiast.
-Michael