Okay I need some help figuring out a misfiring issue with my g35. Recently purchased it off some kid that said he swapped the motor out. It's throwing p0300 code, I've already changed all spark plugs and coil packs, fuel rail, fuel injector wiring harness could really use some help

 

Probably a bent pin in a harness somewhere. Unplug the coil packs and ring them all out. One wire will go to the IPDM for power (they are all connected together with the same power wire), other wire is the ground and will each go to the ECM.

Or you can do it the old fashioned way and unplug the coil, put the spark plug back into the coil, hold it 1/4" away from the intake manifold (wear leather gloves) and have someone else crank the motor while you check for spark. Also put the plugs in with the coils unplugged and crank the motor, then pull all the plugs and make sure they're all wet with gas.

That will eliminate spark and fuel for each cylinder as the problem.

The engine does run yes? Does it obviously sound like it's got a misfire?

Here's a link to the FSM, strongly recommend you download all the sections for this car. Document starts at FWD-Forward.

https://www.nicoclub.com/infiniti-service-manuals