[Sly300]

I wouldn't use those on a pre-eagle engine. There's really not a huge power gain with them and they cause clearance problems with the motor mounts on Eagle versions. I put 6.1 headers on my 2011 5.7 and used Solo performance free flow cats. I had to modify the engine mounts for clearance and the lower reinforcement bracket on the alternator had to be eliminated. The added a nice mellow sound but I might have gotten 10 hp?
The main reason I installed them was to prepare a little more flow for the supercharger that's going on soon.

 

[Sly300]

Ok let’s try and sort this out:
6.1 exhaust manifolds require 6.1 mid pipes
6.1 mids do not match 5.7 catback (this is why hemi_57 explained a method to adapt the two so you won’t need to spend $ on a stock or aftermarket 6.1 catback). You could purchase whatever 6.1 catback you want or adapt the current 5.7 catback as described by hemi_57.
If money is tight and/or simplicity is desirable, some JBA shorties (#1964) will perform similar to the 6.1 shorty manifolds and you would not need any changes to your current exhaust system.

I have the 6.1 manifolds and mids (Solo performance 3") on my 5.7 (eagle) cat back and they fit just fine. I tig welded the connections becuause the 3" mids fit perfectly over the pipe (not inside, but outside) and I didn't want leaks.