Which head/combustion chamber is better for performance? Speed master aluminum LA or aftermarket magnum iron head?

StreetSleeper said

Either KB-107 or Speed Pro H116cp

Cam would be smaller than comp cams XE 268 for the street build

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With that cam any of the magnum style heads would work well. Even the stock magnum heads with a good valve job very minimal chamber and bowl work would be a good choice. Check out Charles Serviedo (spelling?) youtube videos about small block mopar heads.