I would recommend opening the drains on your manifolds and block next time while you are flushing it to clean out any sediment that has built up. You may need to unclog them using an icepick or small screwdriver.

 

I flush my engine with Salt-Away, available at West Marine. It neutralizes the salt and leaves a protective barrier. After flushing the engine I rinse the boat and trailer with it, especially the hubs and wheel studs. Had the original manifolds on a 7 year old 350 Bravo MAG and they were still going when I sold the boat.