The old coils will not be 620 springs, not even close. To obtain a one inch drop you need to cut 1/2 coil out of the springs. I would cut new springs to control the tighter tire to fender clearance. I cut 1/2 coil out of street progressive rate (350-480) springs and they work great. I wish I would have cut 3/4 of a coil because it would have matched up with the mid-eye springs better.
Wayne....As much fun as I'm having with my new coil spring install, a 1" drop is really tempting right now! IMO, one needs to consider spring integrity when cutting, such as load strength, coil "alignment" matching, ie., the bottom spiral matches exactly on the spring perch as it originally did, the insulators on top fitting properly. This would be the case where I would buy a new set with the drop. They are new then and there is no doubt as to wear characteristics (old coils don't look wore out.) This would be the one instance I would encourage someone to purchase new instead of modifying an exsisting piece. And I love to rebuild my stuff in order to save money. I think you would be ahead to buy new coils. Then have fun installing the darned things! I still don't have mine in yet....
Cutting the coil will increase the spring rate, but will not get you close to a 620# spring for the amount you would be cutting out. Over the years, I have swapped my springs out atleast 3 times. First I tried Boss 302 springs. Way too long. I had to cut 1 1/2 to 2 coils to get them to fit. A few years later I got 620 springs from Canadian Mustang. They were too long until I pulled a doofus and cut too much off. I then got a set from Mustangs Plus and they fit perfect.
I know $'s are $'s, but you will be far better off buying a new set of springs. 620's are typically cheaper than "stock replacements" because they are so popular.
I beleive Shelby did not change spring rates from the stock GT springs. (480#? comes to mind, but that is a guess)