Out of the 3, having the body there is the most important. The engine does not have to be the final one that will go into the car, an old, similar engine is fine for mock up purposes. But you need to figure out how much room you need in the engine room for fan and distributor clearance on both ends. The tires and wheels are just there to establish wheelbase and can be any old wheels and tires. Some people have even cut round circles out of plywood to simulate the diameter of the tires they plan on running and using those for mockup.

You have to think of this process as if you were building a house. You wouldn't just start laying block for the basement without knowing what kind of house would go on top of that foundation. Similarly, you wouldn't build the frame without knowing how long and wide it needs to be to fit the intended body, or how much kickup it needs to sit at the ride height you want.

Don