You talking about 18* Big Chiefs? If so I don't think you would gain anything on a 4.9 vs a 4.84.
11* Maybe, but I wouldn't spend the money to try it. You's be better off with a true 4.9 bs head or even better a 5.0 head on a 4.9" motor.
One of our classes has an iron block rule which is why they don't use a 5.0 alumium block.