I have a 06 5.7 rt but I want to put a 6.4 srt hemi into my car can it be done and what is the down side of doing it

 

It can be done, but it won't be cheap nor do you likely have the equipment to do it. The first gen CAN bus PCM you have isn't designed to run a VVT engine. Easy route would be to dissemble the engine, swap the crankshaft tone ring for an earlier one (so your PCM can read it) and lock the VVT phaser in front of the cam. Then you can get a custom tune to run it, however this negates the advantage it has with VVT over just taking your 5.7 or a 6.1 and stroking it out to a 392 , though you can still run the MDS with the stock cam (automatic 392's get MDS from the factory). Next option is to (as mentioned) build a 392 stroker out of your 5.7 or a 6.1 (if you can find one) and everything goes back together the way it's supposed to without heavy modification/adaptation. Last option is chat with a place like Muscle Motors, they've done this swap; I was quoted $13,000 (2yrs ago, I'm sure the price has gone up by now) for harness and computer alterations as well as the cost of a new 392 crate engine since they won't install a used engine. I was told there is also some functionality loss mating early CAN bus to later CAN bus, like bye bye cruise control.
So to summarize:
Option 1 is only really worth it if you can get a helluva deal on a 392 engine but negates one of it's design features
Option 2 is the easiest and most likely the cheapest
Option 3 is the most expensive and may cost you some things you take for granted