318 or 273?

Answer depends on the car (so many posters keep that secret). In a 63-66, a 318 will be easier since a 62-66 V-8 oil pan is needed to clear the steering, and won't fit a 360. If looking for a used engine, 318's are everywhere. I saw several 80's Mopars at PickNPull a week ago. Also consider a Magnum 318 (5.2L), which is a cheap way to get MPFI and overdrive if you take the tranny (must re-shape car's tunnel). If power and after-market parts is your goal, a 360 (or 5.9L Magnum) is better.