Well it won't void your warranty unless they are able to prove that your new exhaust has caused the vehicle to require the warranty work.
I see you have a 5.7L check out the classifieds there is a nice muffler being sold in there. Best way to hear what it may sound like is youtube. Also search exhaust in this forum and you should get numerous hits.
It is still a hemi motor that has been in multiple vehicles. Just look under there and take some measurements or take it to a muffler shop they will be able to help you out.