The stock "S" springs are adjustable, might only be compression but it's something to try first. Also finish your aero first, and based on FFR's wind tunnel testing, account for the added weight at speed when choosing a spring. I'm not surprised your front end was loose, without a splitter or any rad ducting it was a big air scoop. Any springs you buy should work on a future "better" shock, so there's no harm in playing with spring rates now. Are you running a front sway bar?