* The specific block casting: Different years and even different production runs of the 318 block have varying thicknesses in the cylinder walls. Some blocks are "thicker" and allow for more boring than others. A machine shop will need to inspect the block to determine its suitability and maximum bore size.
* Desired piston-to-wall clearance: Too little clearance leads to piston seizure; too much reduces compression and power. The machinist needs to calculate the appropriate clearance for the chosen piston and rings.
* Availability of pistons: Once the maximum bore size is determined, you need to find pistons that fit. Oversized pistons are available, but their availability varies depending on the engine's year and the bore size.
In short: You need to take your 318 block to a reputable machine shop specializing in engine rebuilding. They will inspect the block, determine the maximum safe bore size, and advise you on available piston options. They're the only ones who can accurately tell you how much you can bore it out. Trying to guess and exceeding the safe limit will weaken the block and risk catastrophic failure.
