Rick, I think you've got the wrong casting number...the Ford number should be something like D1ZX (D for '70s, 4 for 4th year, Z for Mustang, X for Ford Muscle Parts)...but Pat Ganal's book notes that all 428s sold after '74 were bored out 391 truck blocks, characterized by the vertical outside ribs on the block and a 3/8" pipe plug on the lower side of the block for the truck
oil return line...