Hah, if you think an ecotec 4cyl is going to produce 600hp with just a turbo, you're out of your mind. For one, the torque of that engine is so weak it's not funny. A Grand Am is a bit heavier than the sunfire/cavalier, and it would be slower yet. Also, the engine is built for gas milage. If you add a turbo, do you think it's going to get good gas milage still? No. You won't see 600hp with just the turbo, because the engine will grenade long before it gets there. 300hp if you're lucky, and that won't last long.
I wasn't serious about the big block, as I thought I made clear with my post in pointing out why it wouldn't work. But I guess you didn't quite understand me.
If you want a bigger, more powerful engine, get the V6, or look into squeezing an LS1 in there. I'm not 100% sure, but I think that it's possible to fit an LS1 in without any major work. Probably need a new transmission, of course, but it would definately give you the power you want.