will LS1 5.7L Engine fit in a 2006 Pontiac G6

Approaching this from a non bias point of view. I would think with enough modification to the sub frame (For engine mounts) You may be able to fit the engine, the transmission may be a problem though. If you gut the G6, pull the motor trans, exhaust. You do have some what of a good tunnel to run a driveshaft, but not much room for a tranny to sit. I'm pretty sure the bell housing would just sit against the firewall. At that point I'm not even sure if the front of the car will even clear the motor. You might be getting into custom radiators or a relocation as well as firewall work.

So lets say you can fit the motor and trans in, or they will fit (I'm not sure how big the LS1 is with a trans but judging by google images it's pretty big). Once you get the drive shaft in place you'll have to find some kind of rear sub frame built to handle the power and torque while holding a differential in place. You may be able to get that off the saab 9-3 which is built off of the same platform, But saab did change it up quite a bit so it may not be a complete bolt up. At this point you would have the relocate the fuel tank to make room for the drive-shaft.

After that's all figured out I'm not sure how you would run the exhaust. The tunnel is now housing a drive-shaft, either run the exhaust really close to it or risk scrapping it running it elsewhere.

Just telling you from a point of veiw in having heavily modding cars in the past, the swap you want to do just isn't worth the time/money. Parts alone would easily take you over ten grand. It's more than just a motor swap, there's a new trans needed, drive shaft, and rear end. What would you do, live axle or IRS? What computer would you get to run the motor? Would it then run your factory gauges?

Other parts to consider would be upgraded brakes, bigger tires, stronger suspension to hold the extra weight, custom headers and catback, and we haven't even touched all the modding needed to be done.

You'd be better off doing this for the G6 if you really want to keep it and make it fast... If you have the 3.5 litre, look into the forged internals that are available for it. Swap the crank, rods, pistons, cam, pushrods and rockers for upgraded parts, throw a custom turbo kit on, find a good transmission shop to build up the trans and get a tune from someone like Chris White. It will be cheaper and there will be minimum modding needed.

Or, sell the G6 a and start with a V8 platform like a Camaro or Mustang. Just my .02