after running 1 season with the oval ports, in the 9.30-40 range, depending on air, with a best of a 9.298 et, at 143-145, again, depending on air, for the next season, I did nothing else, except change to a set of mildy ported Canfield 310's, same intake, except ported to a rectangle port, which was a Victor-O 4500, same cam, same everything, only change was the heads, the car ultimately ran 8.989 at 150, with a string of 9.0's. I didn't dyno the engine after the head swap. But, I did dyno it later, and it made about 70 more hp than the ovals.
"Thats about what I would figure..you picked up 3 1/2 - 10ths, and it took 70 hp,, thats 20 hp per 10th"