What’s the biggest engine I can fit in my car?

That all depends on how much money you want to spend and the modifications you’re willing to do. With the vehicle in stock configuration your limited to what you can do.

Answer not posting

Remember those days when you could stuff a 427 into a Chevy II? Well that can't be done anymore because your car came with a computer that's set to a particular engine. Your 22 year old Pontiac is good with its existing engine and changing that will necessitate a computer reprogram. That's possible, but the cost will likely exceed the value of the car by a factor of four.

you need a donor monte carlo 2004-2007 5.3 v8 and the cradle and computer with it. quite A HUGE job.