magnaflow makes a nice high flow cat with the pipes that attach to our manifolds. I bought one years ago and couldnt be happier with it!
as far as exhaust im looking to do the same thing
I have dual exhaust now, I wish at this point i left the factory exhaust on and just changed the cat and stock muffler
If i were you, to keep the torque up, go with a 2.5-3 inch single pipe system
as far as mufflers its your choice. Dont go with flowmaster, they are loud, annoying and and dont flow very good.
Id suggest magnaflow (great quality and sound) dynomax (good value and sound)
I heard some guys running corsa mufflers and borla...with good results
I love the sound of magnaflows. I have them on my 440 charger and love the sound (once they broke in)
here is what im shooting for with my truck
https://www.youtube.com/watch?v=SJOCBq27mXg
skip to 2:20
thats the factory exhaust pipe with mag high flow cat and mag muffler
sounds so good...very jealous of his sound