New Engine
 

Ok so im considering buying a new engine for my 2006 Monte Carlo 3.9L LT. I wanted to know if i bought the 2006 5.3 L engine would it fit in my car and would i need to buy other parts for it.

It would but you would have to change everything.

If I was in your shoes I would just sell the one I have and look for an SS. Much simpler.

best bet is to have a donor car to take every thing from it to put it in yours.
I have been looking into it my self I ran the parts I need.
engine trans ecu tcm bcu and and the wire harness to go with it to run the system and the other o2 sensors and a stronger fuel pump as well.
plus the engine cradle as well if the 5.3 engine wont line up with the 3.5/3.9 engine mount location. I am not sure if they do or not that part is hanging me up.
but I still like the 3.5/3.9 pepy of the engine.
if I did some thing like that I have to make sure to have a reliable dd before digging into a prodject like that plus all the odd and ends that run into a swap like that.
I am not saying don't do it expect it to be laid up for a while and time consuming of the swap.
it would be one of a kind to you if you did some thing like that it is possible to do but time and all the wiring

Agreed, the time/cost of the swap just doesn't justify the work when you can buy a factory ls4 car with the same body style. Your options are even better if you don't mind branching out into a gp or impala to get the ls4.

I totally get engine swaps on cars where the engine wasn't available originally, but the huge depreciation on gm cars makes it hard to resist just buying a whole functioning car.

As soon as I catch an ls4 in the junkyard I'm doing the swap