Considering that there are TT Terminators on the street with 750 RWHP, Z06's with long tubes and cams making 550 at the rears, Vipers with blowers, and any number of street rods with over 500 at the hides, I don't think a request of 600hp is out of line. If it gets scary put a block of wood under the "Wow Pedal".
Stage II heads as stated above, 10.5:1 compression, .600 plus lift cam should get you on your way. If you have never built an FE before, consider talking to someone like Keith Craft. Forged crank is a good idea.
Here is 667 lb-ft in action
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