Aluminum vs Iron compression on pump gas

Is this a 4" arm engine? Stay away from the Crosswind if that is your goal. It cant move enough air.

I'd go with the RPM Magnums with the smaller chamber. The aluminum will help with the pump fuel. But, you could run a much bigger cam if this is a 4" stroke engine. I have a hard time seeing you getting anywhere near 600hp with a 256° cam, regardless of heads or lift. Like was said, smaller chambers, good quench, mean less total timing.