The difference between a 305ci engine and a 350ci engine is the size of the engine displacement in cubic inches. Engine displacement is the difference in cylinder volume from when the cylinder piston is at top dead center compared to when it is at bottom dead center multiplied by the number of cylinders in the engine.
Both 305ci and 350ci engines measure their single cylinder displacement by multiplying the cylinder bore size by the distance the cylinder travels, which is referred to as stroke. By altering the size of the bore or stroke, you can change the displacement of the engine. A 350ci engine would have a larger bore or stroke, or combination to make it 45ci larger than the displacement of a 305ci engine. Generally, having a larger displacement allows a larger amount of air and fuel to enter the cylinder and therefore more engine power. However, there are many things that factor into the power of an automobile, including the overall weight, the type of air and fuel delivery and the valve configuration.
Engine displacement can also commonly be displayed in metric terms, such and cubic centimeters or liters.
After converting to metric measurements of cylinder volume, a 305ci engine is the same as a 5.0 liter engine. A 350ci engine would convert to a 5.7 liter engine since engine manufacturers round numbers.