The block is the foundation of an engine, the main component to which every other part is attached, and it better be right, especially if it’s a high horsepower performance motor, or you can quickly end up with a very expensive boat anchor. You can base your performance engine on a used OE cylinder block, but you have many more options and a much greater chance for a successful build with an aftermarket performance engine block.
Some engine builders prefer a "seasoned" original engine block, but dimensional accuracy is critical in a performance block, and an old block’s accumulated wear may prevent you from attaining this degree of precision. Even if you can, the cost of checking for cracks and the various machining operations to make the block right could end up costing you more than a new block. But there are many other reasons a new performance engine block is a better choice.
Aftermarket engine block makers analyze the factory product and they know the weaknesses and where improvements can be made. Plus they’re not under cost and weight restrictions like the OE designers. As a result, you can get blocks made with more material to prevent bore distortion and maintain head gasket seal, and billet steel splayed 4-bolt main caps for greater strength. The factory cooling and/or oiling systems are less than ideal for performance on some engines. Blocks are available with larger water jackets for improved coolant circulation and re-engineered oil galleries and oil feed holes to increase oil flow.