I have a 71 Big-Block hot rod with a Vette Brakes and Products composite spring. It's 360 lbs/in. I tried a 330 for a while, but it wasn't stiff enough for the engine's torque output.
If you have a stock Big-Block and the car used for easy cruising, the 330 would be a good choice. If it's a Small-Block car, I'd go with the 300.