I dont care about it being lowered or and stock level at all, just COMFORTABLE!
That's very simple!
Get the 400lbs/in 4cylinder springs. They are also what is now sold at the parts store.
But, be WARNED, the springs are taller (because of their lower rate). So, they will be a bigger pain to install. Good luck trying to install those tall springs unless you drop the FCA and/or use the OTC tool.
Then, add good struts/shocks! IMHO, forget the KYB junk. For the money, get the Tokico 5-way adjustable struts, and set them on a soft setting.
The problem with the 4cyl springs in a GT is that the car will be lowered, and the spring rate will not be higher (as it
should be). Your best bet to prevent bottoming out the suspension by accident would be to also put in the Steeda 1/4" springs spacers (like I have). They will raise the fender height by ~0.4". You can buy them by calling Steeda directly and asking. They don't have the listed separately in their catalog or web page.
Or, someone may have a set they are not using, since they come with the Steeda X2 ball joints. And, many people just use the ball joints to help with their bumpsteer. So, you may want to post a WTB in the classifieds.
You can see the Steeda spacer on top of the isolator, along with OTC tool and it's use in the following web page:
Spring install on a Mustang using the OTC spring tool
Good Luck!