Don't use used springs. This is just a general rule. They are going to be worn, rusted, cracked, chipped, broken, or compromised in some way more than likely.
If your 455 is installed without A/C equipment, the standard new small block V8 springs will probably be right on the money. I imagine the V6 springs are sagging a lot- a new set of springs for your car ordered as if it had been small block V8 equipped will give you the lift so the car looks stock. If you are in to the lowering thing, I would consult a spring/suspension specialist. The G body front suspension is not exactly a hot handling/performance item, and most often if you are just going for looks, you will diminish its driveability even more by modifying it with parts that are guesswork.
-UT-