I found this on ChargerForumz.com that a guy did 5.7 to 6.1 intake manifold swap. He went through this company: http://www.inertiamotorsports.com/motors.html
The upgrade kit is towards the bottom of the page its $425 for the kit and then you have to supply the manifold and do a couple other things like build a CAI for it, but its very possible.
If I remember correctly I think the guys name was Stu at that company.
There is a picture of the engine with the swap but the file is to large for me to post the picture.. If someone else would like to post it here I have the link.
This is the setup the guy used with the swap:
FRI Ported & Polished 6.1L Manifold
FRI 5.7L Extreme Cylinder Heads
FRI Littleboy Camshaft