When the piston crosses a point just past bottom dead center (BDC) the intake valve closes, the piston swings up the bore and the mixture is compressed into the combustion chamber. When the piston reaches a point just before TDC the ignition fires and the compressed mixture ignites and burns rapidly. As the mixture burns it expands, and the force created pushes the piston back down the bore with great force. This force drives the crankshaft, which produces torque. And when the air/fuel mixture is just right, big horsepower results. Finally, the piston reaches BDC, the exhaust valve starts to open and on the final upstroke the burned gasses are expelled and the 4-stroke cycle begins again.
Despite the pumping capabilities (based on large or small displacement), you must remember that it's the chemical reaction of the fuel and air that releases the energy to make horsepower. The pump (our engines) simply contains and harnesses the release of heat and energy. So yes, a larger pump is one way to increase power, provided it can move more air, more fuel, and provided the mixture is burned efficiently. It's also important to remember that by increasing the bore and stroke without increasing the size of the combustion chamber will yield a higher compression ratio and that always has to be accounted for.
Bore & Stroke Basics
There are two important dimensions that determine the displacement (cubic inch size) of any engine and they are bore and stroke. Bore refers to the diameter of the cylinder bore, while stroke refers to the distance the piston travels in the bore. If you use this equation: bore x bore x stroke x .7854 x number of cylinders, you can figure out the displacement of any piston-type engine.
You should know that there are benefits to increasing both the bore and the stroke in an engine, but big jumps in engine size will come after the stroke is increased.
Increasing the bore is beneficial because it opens up the space between the intake valve and the edge of the cylinder bore. This unshrouds the valve and helps flow. By increasing the stroke you increase the swept area that the pistons travel and this yields a much larger increase in engine size when compared to simply boring a block. For instance, overboring a 302 block by .030-inch adds four cubic inches. Yet, stroking the engine .400-inch (less than half an inch) adds 39 cubic inches.
Due to the dimensions of the Ford small-block, we are limited to the amount of boring and stroking that can be done. Blocks are cast with only so much wall thickness, which limits bore size. As for stroke, that's limited by the space available in the crankcase. When stroke is increased, the connecting rod's big end is essentially moved further away from the crank centerline and clearance between the connecting rod big end and the oil pan rail is reduced. In other words, the journals spin in a larger diameter circle.
So, in most cases, it is necessary to grind portions of the block and the pan rail to gain the necessary clearance. This is not a big deal and can be done with a small grinding tool like a Dremel.
Does Size Matter?
In a word, yes, size does matter. And with the variety of aftermarket internals on the market you can choose between an endless variety of cubic inch displacements. In most cases bigger is better, as long as the all the parts support the combination.
However, the intended use of the engine plays a huge role and can effect the end user's performance, reliability and longevity. Some factors to consider include rpm range, type of use (drag racing, towing, street performance, etc.) and the expected life of the engine. When dealing with the standard 8.2-inch block, the most popular stroker kits are the 317, 331 and the 347. All of them feature different strokes using a .030-inch overbore. Popular choices for your 351 are 377, 392 and 408.
Since you can only bore the Ford small-block so far (usually .060-inch max), the major increase in cubic inch must come from extended crank stroke. But upstroking a crankshaft is not a simple thing. If you added stroke and changed nothing else, the pistons would stick out of the top of the block and crash violently into the cylinder heads. On the down stroke the counterweights would slam into the skirts of the pistons. So when upstroking an engine it is necessary to change the connecting rods and pistons, too.