Looks like you're not getting anyone who has tried this so I'll pass along a comment from a respected source that was on the archives of another forum...
=======================================================
Manifolds are tough to properly weld. Here's why:
1) The metal is cast and heating it will make it tend to warp, thus the fit after the weld is iffy.
2) Two different metals (the manifold, and the Welding rod) are hard to get to stick together with any degree of permanence. As the engine warms up the two metals heat up at different rates. This causes one to expand more than the other and try to come apart.