You should be able to reach the 500 HP level fairly easy with the 906s, i ported mine but kept the stock size valves, the heads flowed 268/198, & there was alot left to port if i went crazy. Dave is right on that Demon carb, you'll be way better off with the Mighty over the Speed in your combo.

75 Duster, 451 10.87 @ 123.58 NA 97 Z28 6sp., 12.01 @ 115 on a 100 shot 71 Swinger. 360 magnum. 12.58 @ 105 78 cutlass, 469 BBC. 12.70 @ 108 on street tires. http://www.youtube.com/watch?v=Z2kqNmMfheU