Need Help W/ Suspension!!!

Hey, it's a 1991 Mustang Hatchback. 17x8 AFS Cobra R Rims with 245x45 17's on the front. The tires on the front are a little taller than the 255x40's in the back, but when I actually put those on the front I can now atleast make it in my drive way without scraping the exhaust. And that could of only been what a 1/8-1/4"? Anyway, I still had the problem before when i had 255x40's on the front too.

Any one know the spring rate of kenny brown springs in the rear? I tried to get info off their website and couldn't find hardly any products they sell. Searched the web, and they dont have an email address unless I overlooked it. I saw the phone number but I can't call long distance. MM wants to know the spring rate to match up the front coil overs.