LT's + an e-cutout...
You can do longtubes, aftermarket y-pipe, cutout, stock catback.

Do not run the stock cat-back, it is a major restriction even on an otherwise stock vehicle. Only run it if you are very determined to have the ultimate sleeper. There are a few cat-backs that flow well yet are pretty quiet, especially if you run high flow cats. SLP dual/dual, Hooker and Magnaflow.

Do not run the stock cat-back, it is a major restriction even on an otherwise stock vehicle. Only run it if you are very determined to have the ultimate sleeper. There are a few cat-backs that flow well yet are pretty quiet, especially if you run high flow cats. SLP dual/dual, Hooker and Magnaflow.
Man, holy run-on.
Man, holy run-on.
If by "aftermarket system" you mean a cat-back, yes then. You need to match the Y-pipe to the headers but any and every Y-pipe should match up to any and every cat-back.
I recommend hooker. The tips look alright, it's not very loud, flows well (should make plenty of power through it) and is inexpensive. <$300 last I checked.

