360 stock intake manifold on a stock 318?

Air flows from higher pressure to lower pressure. There is no such thing as vacuum, only pressure.

The issue with a mismatch on ports is turbulence. The turbulence increases the boundary region and causes the free laminar flow region to become reduced. This in turn increases overall resistance to flow volume.

Resistance to flow in an air pump reduces efficiency and in an internal combustion engine, reduces HP and performance.

Suck air through a smooth straw and then one of the same diameter that is corrugated. You will find that the corrugated one requires more energy to pull the same volume of air through.

The same is true on an engine, more energy required to pull a specific volume air means less energy output from the system to drive the wheels. (reduced performance/HP).

It's all just simple physics.