The V8 engine is larger and heavier than the stock engine, so the chassis and suspension would need to be reinforced to handle the extra weight. The transmission and driveshaft would also need to be upgraded to handle the increased power and torque of the V8 engine.
In addition to the mechanical modifications, the electrical system and fuel system would also need to be upgraded to support the V8 engine.
Overall, putting a V8 engine into a 1996 Nissan Hardbody is a complex and expensive process, and it is not recommended for the average person.