I'm not saying it can't be welded...I'm saying the weld won't hold. Think of it this way: the manifold gets heat treated every time the engine starts. It amounts to thousands of heat treats, and eventually the grain structure reaches a point that the original grain lattice cannot be recovered with further heat treating.
I've also welded blocks, heads, crankshafts, intakes, you name it. As a young gearhead teen I could not figure out why I could never weld exhaust manifolds with any success. Weld looked beautiful...and cracked. Do it again...cracked again. Later, I studied metallurgy in school, and learned that exhaust manifolds are hardened to the point of no return! The grain structure of the weld can never be matched to the over-treated structure of the manifold, no matter how much you preheat, cool controlled, or otherwise prep the weld. It will end up brittle. A brittle weld may, possibly, last on a grate, but it will never take the load of that stud on the manifold.
Another way to put it...I could weld that for free...and I have saved many a lost part from the grave for no other reason than the challenge of trying it...but I still would not attempt Welding your manifold. It has no chance of success.