ideal spring rate for 68 impala

[fastlane]

I had cut springs on my 68 for years some how a front spring broke, I have decided to go with the qua1 coilovers, the area is a 396 iron head with air, I am thinking 450? The only reason I question the 450 is because I run the car very low in the front, there main purpose is for ride height. Thanks for any input
Lane