Contact
www.maximummotorsports.com
Chuck is an excellent guy and can give you personal advice.
They carry many products related to road race your stang
H&R springs are a german brand and they come in different sets to suit your needs. I actually have the hardest ones (950-1050lbs) and they aren't as unconfortable as I thought because they are progressive.
Another option are "C" type of springs that Ford Racing sales. I also have them, wich are BTW softer than the others.
lito