If the piece that was broken off the manifold has a load on it, the JB weld will not last. It has no resistance to vibration and no inherent strength and will fail shortly. If it survives a week, I'll be surprised.

To weld the manifold, the JB weld will have to be completely removed. Shouldn't be a big deal anyway. You don't just lay a bead on top of a crack, the casting on both sides of the crack will have to be V'ed out so the weld rod can penetrate with both sides of the casting and the weld bead can be built up to fill the void if the outer surface needs to be ground back down. Any machined surface involved in the weld effected area will need to be re-machined flat. Gene