Idle (500 rpm) to about 4500 is a good rpm range for a 400. The thing was built for trucks and made it's way into the large cars, I.E. crown vics and town cars. They are great torque motors. I'd call comp cams or lunati, and see what they say about grind. Give them the horsepower you want with your setup, at the rpm you want. With gas prices the way they are I'd say a hp peak of 3k with most of the torque under 2k would be a great street engine.