Depending on if your dads engine is setup to leverage the extra flow, it could see a much bigger gain than that. The Trick Flow heads flow about the same as my fully ported Edelbrocks (293cfm).
Not an apples to apples comparo, but my old 360 with OOTB Edelbrocks and a 509 MP cam ran 12.5...I rebuilt that motor with a 4 inch crank (410 cubes) and had the heads ported, along with a street friendly 10.8:1 compression....that motor went 10.7 all day every day with a "relatively mild" cam, with a best of 10.57.
