Can you put a 84 firebird 4 speed in a 79 firebird automatic?

Yes, you can put an 84 Firebird 4-speed transmission into a 79 Firebird that originally had an automatic transmission, but it will require some modifications. You'll need to address the differences in the mounting points, linkage, and possibly the driveshaft. Additionally, you'll need to convert the car to a manual setup, which involves installing a clutch pedal and a master cylinder. It's a project that can be done with the right tools and knowledge but may require some fabrication.