some 318's had forged cranks but the majority were cast. 67 did for sure and some 68's. sometime in 68, mopar switched to cast cranks. the old poly engines had forged cranks and the bigger dodge trucks had a special truck crank that was super strong.
The 318 forged cranks (standard) were basically the same as the 273 cranks. only slight differences. the 340 was the next step up and it was stronger than the 273 crank. BUT, the 318-3 truck crank was the strongest of them all, it had the 8 bolt flange for the flywheel. I do not know how to tell a 340 crank by looking at it, I have some pictures all I know is that if you put the 340 crank next to the 273 crank, you will see a noticable difference. If you can find a number on it, might help.