I've got a magnaflow magnapak on my 4.7. Its 14" long and 4" round. Its got a nice rumble at idle and super loud at WOT. I was just given a new Aero Turbine Muffler but not sure what model it is. I think its the 3030. I may be putting it on as soon as i can get my welding machine back.
Here's the magnapak video before my header install. I'll have to get a new HD video up with the new setup soon.