340 main caps are also slightly taller than the 318 caps, although they can be interchanged. If I remember correctly, the 340 caps are about .250 taller than the 318 caps.
Well I went out to do a little checking. The heavier duty caps are taller and the bolt holes are further apart. The bolts that came with the larger caps do screw into the 318 block I have but the bolts seem to be too long. Since the larger caps are taller the bolts are also longer and probably wouldn't work on the 318 block. It looks like the taller caps will fit on the 318 but since the holes are further apart they must not be 340 caps??? So are the larger caps 360?? The larger (taller) caps also have a bigger bearing (larger diameter).