The stock exhaust system creates a lot of cabin drone, which can make conversations difficult. The Corsa cat-back exhaust will eliminate this drone. With the Corsa and stock cat/headers, your Viper will be very quiet below 3000 RPM's in the cabin. As a bonus, above 3000 RPM's the car will sound so good you may want to turn off the radio. I would recommend this as your first step. The Viper has so much low end torque, you really don't need to go over 3000 RPM's when cruising. I doubt you will need more soundproofing, but it you do the door panels can be removed in minutes... I would put some sound damping material in both door panels. Then lift the carpets and do the same. I guess another spot could be the trunk, same process.
Later
P.S. - I would NOT put any additional sound material under the hood. Too much heat is generated, and you need all the open space/air flow you can get to cool the beast within....
