Last year of the forged steel crank in the 318 (and 273) was 1967. These cranks were balanced for the light duty rods. I've heard some maintain that the 3/4 ton and above trucks with 318s had forged cranks, but I have not been able to verify.
http://www.allpar.com/mopar/318.html
If you have a forged crank and heavy duty rods, it would seem that you have a 340 (68-72) crank. Change from forged to cast cranks was a running change made as a running change sometime around February 1972. Here are the 340 crank part numbers.
1968-1971: forged crank, part number 2532457, 2128869, or 2843868; service part number, 2843868
1972-73: cast crank, 3462387, 3658393, or 3751841 (some 1974-86 cast cranks use the same 3462387 casting number as the 1973 340 crank); service part number 3751162
From what I've seen someone has lavished some attention on your 318. Hope you don't have to throw out much.