the 392 is a stroker kit that replaces the pistons, crankshaft, rods, etc. It brings the 5.7L (345) to a 6.4L (392). It costs around 3k + labor. Of course with this you probably want to open up the intake and exhaust as well since you'll have more demand. From what I've read this should bring it up to around 400+rwhp. If you go this route you might want to find a stronger limited slip if you plan on upgrading further on down the road and drive it hard. There is one made by Quaife that is designed for the spirited driver. It has other features that warrant getting it instead of it just being stronger, good idea would be to research it. That will set back about another 2k for a total of around 5k + labor. Which. in my opinion, would be better than putting in the srt 8.
FYI: For all you srt 8 owners, there is also a stroker kit to bring that 6.1L (370) up to a 6.9L (426) for the same price.