the cam in my SS is a .608/.612 duration @ 0.50 .242/.248, if your gonna run a cam that size I wouldn't mill the heads unless your gonna change out the pistons also you'll need a 4.30 or higher gear ratio, I too have an M6, I'm running 4:56 gears, so you know I've gone through 3 transmissions, 1 rear diff, 4 sets of ring and pinion gears, 1 engine block, and countless other crap.
so make your cam choice based on how often your gonna drive this car, remember gas isn't going down anytime soon, a cam that size in a DD is gonna put a strain on your wallet over time along with broken parts when your out beating on it for fun.