Sorry to say it, but this has been asked many times and you really should search... BUT from my learnings from asking the exact same question basically lol, here is what i know...
Ok, so first off, duals is not really a good idea for this lil 6cyl. Simply doesnt move enough exhaust and wont sound as good from what i hear. I was told that the max pipe size you would want to run on these engines is 2.5", and that 2.25" is the best size you should go with (Not duals). Stock piping should be 2", but idk what the magnaflow kit gave you.
I've heard that flowmasters are the muffler to go with for deep tones. The flowmaster origonal 40 is supposed to have a real nice rumble at idle and roar when punched. I myself am going to redo my exhaust with 2.25" pipes and a single flowmaster 40 muffler. Was planning to run it out the sides, but heard that it might sound better going out the back.... Can someone maybe clear up what the sound difference would be with different tips and exits for us both please?
Either way, if you are deadset on running dual exhaust, i would just go with "fake" duals and get a SI/DO (Single in/Dual out) muffler. Have it maybe something like 2.5" pipe to the muffler and 2" pipes after. Idk how loud you want to go, but if you want a deep muffler i would go flowmaster 40. But that may be too loud for your liking if you are going for a quieter kind of deep rumble.
Hope this helps some. Use the search button and look for more info. I know one of the topics i started a while back when i first joined had pretty much the same question lol.