Mark tells us the humble 289 C5AE cylinder-head casting with its 53-57cc chambers makes an excellent factory iron casting for port work. Because Hi-Po head castings are valuable, we don't suggest you port them. However, 2V/4V C5AE and C6AE castings are good portable castings because there's lots of iron to work with. The casting walls are nice and thick, which allows you to make these heads even better than 351W castings, according to Mark. The smaller chambers and thick walls make them good candidates.