In addition to more cubes, porting the GT-40s is another sleeper element of this engine upgrade. We worked a barter deal for a wicked port job on the swap-meet heads. After opening up the runners (and going as far in as possible to remove excess casting flash), Rob Bieschke performed a five-angle-intake (35, 45/back-cut to 37, 55, 65 and 75 degrees) and three-angle-exhaust (35, 45, 55 degrees) valve job; “bowl-hogged” the intake ports on both heads; and rebuilt the them with new valve guides, seals, valvesprings, and their associated hardware. As Rocco notes, there certainly are better cylinder heads that can be used on a 331. However, when you’re keeping funds close and aren’t out to set the world on fire with your ‘Stang, the iron heads are a great bang for the buck.