Not all 318 rods are light duty, only the early ones were light duty, the later ones are the same as the 340's and most of the 74/75 up 318's use the heavier 360 rods.
Here is a brief description of the part numbers,
Yr, engine, pin type, part number, forging number
64-71, 273/318, Floating, 2406785, 1518699
72-73, 318, Floating, 2899495, 2899496
74-86, 318, Pressed, 3751015, 3418645
68-73, 340, Floating, 2899495, 2899496
71-86, 360, Pressed, 3751015, 3418645
There are also a lot of 2899495 and 2899496 rods that are not floaters, and I have also found several 70 and 71 318's with this rod as well.
Also the 72 and 73 340's were cast crank motors, 71 was the last year for the internally balanced 340
Here is a brief description of the part numbers,
Yr, engine, pin type, part number, forging number
64-71, 273/318, Floating, 2406785, 1518699
72-73, 318, Floating, 2899495, 2899496
74-86, 318, Pressed, 3751015, 3418645
68-73, 340, Floating, 2899495, 2899496
71-86, 360, Pressed, 3751015, 3418645
There are also a lot of 2899495 and 2899496 rods that are not floaters, and I have also found several 70 and 71 318's with this rod as well.
Also the 72 and 73 340's were cast crank motors, 71 was the last year for the internally balanced 340
