Bumper has to be removed / partially removed. There are three screws that cover the ballast screws so the headlight assembly needs to be removed completely.
If anyone is curious, this is the part number for the 370z ballast. The ballast I replaced it with was from a 350z unknowingly. It works fine, however the tension release bolt design it a tad different and the metal clasp around the igniter cable fitting did have to be cut just a smidge to fit snugly in the 350z ballast
Basically. A 350z and 370z ballast are interchangeable. To replace the ballast the bumper needed to come off because you have to remove the head light assembly from the car. And the tension release clasp around the connector for the ballast needs to be trimmed if you're using a different ballast than the 370z oem.
No you cannot use an aftermarket HID ballast as the ballast for the 350z and 370z have a grommet that make your light waterproof. This is a unique design and no other aftermarket company makes a ballast in this exact size.