Best reason for not using muriatic [actually hydrochloric] acid for the internal concrete removal is that this acid will also effectively "lighten" your cast iron block as well. Iron [& aluminum] dissolve very well in both dilute & concentrated acids and while you may loosen some of the concrete, it will be almost impossible to neutralize & remove the acid solution from the confines of the block. Then the acid keeps on going & will start perforating the thinner sections of the cast iron....