The problem with waiting on the manifold replacement is it is highly likely to fail, and unlike the aluminum crossover ones, which fail and develop a slow leak that you can nurse home, those ones fail and dump all the coolant out of the engine in about 30 seconds. If you are stuck in heavy traffic, or are somewhere that you can’t pull over right away, you could easily overheat the engine, not to mention the cost of a tow home. For $100 for a manifold and a few hours on a Saturday, replacing that old manifold is well worth the peace of mind.