A 1966 Plymouth Fury III could have come with a variety of engines, but a 318 is a very likely and common fit. It's highly probable a 318 would fit, provided it's from a roughly contemporaneous model year (late '60s, early '70s) and the correct configuration (2-barrel, 4-barrel, etc. matching the original).
However, there might be minor differences in mounts or accessories that require attention during installation. You'd need to check the specifics of the engine and transmission you plan to use against the specifics of the car's existing engine bay. Some swapping may be needed.
