ive done this job before
before welding
i highly recommend preheating the cast iron before welding
dont try and lay on one big single bead
u want to lay down multiple beads ontop of each other as it heaps post-heat
so lay down one small bead then let it cool down to the point u can touch the manifold with your hand then lay down another bead ontop of the bead u already welded
when cooling do not dip the manifold in water to cool down
let the manifold cool down over night on its own
u want it to cool down slowly
one i gone a while back