The difference between a 248 and a 270 is stroke. A 248 has a 3 13/16" stroke and a 270 has a 4" stroke. The rods are the same length so the 270 block is taller. They both have a 3 23/32 bore. A 302 has a 4" stroke and a 4" bore.
270s and 302s are the hot rod choice but a 248 is a good engine and can be built to perform very well. They usually cost less. Bill Fisher sometimes used 248 cranks in builds to reduce piston speed, smooth out the engine and even increase rpms.
