Beyond physical size is the engine design. I believe if I'm not mistaken, these V-6's are 60 degree. That brings the two cylinder banks closer together. So finding a 60 degree V-8 is what you would want. A 90 degree is wider as the cylinder banks are spread apart farther and would make a tight engine compartment even tighter if not impossible to get between the frame rails.
Beyond that, you face a few other challenges:
#1 the transmission/transaxle. Finding a V-8 that will bolt up to the existing tranny. Not only that, but will the stock tranny handle extra HP and torque???
#2 added weight. You may have to do some front end work for the added weight a V-8 will have over a V-6. It may or may not be considerable.
#3 if #1 can't work, the next drivetrain option would be to go with rear wheel drive. Given that these cars have a longitudinally mounted engine in the first place, its a good fit for a rear wheel drive (a topic that has come up before). While finding an engine and tranny to fit is one hurdle, getting a driveshaft and axle for the rear is another. I think one idea that may have come to light is the set up out of a newer 300, with the all wheel drive set up. The platforms are different, so there's no telling if anything will bolt up directly at all, most likely not.
And the biggest challenge #4 MONEY!!!! Talking about a lot of modifying and fabrications.