might checkout Dulcichs' porting articles in the Mopar Muscle archives. (3 part articles/excellent) & iirc there was some area on 516's that should not be touched (short side radius on the ex side?) that is std procedure to be reworked on all other iron OE heads but that they definitely can be made to flow. They do have a bad rep & might be if you touch that area they will flow worse which is maybe why.