I'm not an expert but you don't want a single 3" in / dual 3" out... You can either have a single 3" in/3" out or you need to get a 3" in/ Dual 2.5 out. You want some backpressure on the 5.7L and doing a full 3" exhaust takes away a good bit of your torque(which is another plus of a catback since it helps add some back). I just had a magnaflow put on my truck last week. I ended up going to pep boys and getting a 5X8X20 3 in / 2.5 out magnaflow and taking it to a few different exhaust shops. I picked up the muffler for $79 (military discount) and then I had it installed for 240 and that was with 3 1/2 slanted rolled tips straight out the back.

There are 2 resonators and the actual muffler on the stock system. The resonators look like smaller mufflers and the actual muffler is huge. I had the guy leave the one closest to the motor on since I only had a 20 inch muffler, if I had a 22 or 24in muffler I would have taken it all out but I didn't fell like ordering anything. Overall, it sounds really good... I do a lot of interstate driving and the 3 hour drive I make every other weekend would suck had I gone any louder.

 

Stock exhuast is 409 Stainless already.
Magnaflow is 409 stainless. Gibson is Mild steel.
A complete catback can be done in the driveway by anyone in roughly 2 hours. A muffler will need to be welded by a shop $$$. Take that into consideration. Prob $120 for the muffler plus another $100 for install.

 

I also purchased the 16697 for my 2008 Ram on ebay, seller close out so I got it pretty cheap. Ebay ad stated same thing, that it would fit 2008. Took it to Midas to get installed, muffler didn't match up with the hangers and the front pipe. Midas said they could cut/weld and get it to fit, but I stopped short of doing that.

Called Magnaflow, they flatly denied making anything for 2008 except for the mega cab truck. I asked about a 2007 fitting the 2008, the rep said it would not.

I'm back at square one, have recently been eyeing the MBRP, Corsa and Borla. MBRP dual rear system for $600, Corsa and Borla a bit more. MBRP and Borla are supposedly bolt on, appears fromt the instructions that Corsa requires a cut. Still haven't decided, but leaning toward the MBRP for $600 plus a few bucks to install.

Never gave much consideration to just replacing the muffler, maybe that's the way to go, certainly would be cheaper. Just like the way the dual rear exhaust looks, wish the 2009's dual look had been adopted earlier.

Anyone looking for a 16697 at a very reasonable price please contact me.

 

Dont weld in a muffler or anything
get the Stainless Steal magnaflow system and do it right
go to performancepeddler.com I found it for $418 shipped.
I bought my pipes for my mustang from them, they are legit!