A few reason why you can't.
Cylinder heads are different, intake manifold is different, fuel rail etc etc.
The biggest one being that you can not tune a 2008 Grand Prix, it is a year only computer that has zero aftermarket support from DHP, EFI Live and HP Tuners. It was the first year of the newest CAN-BUS system at the time but the GP was axed the very next year.
So in short, you can't really mod them.