I would start by posting a close-up picture of the parting line on that 340 crank, as well as a picture of the damaged flange you mentioned, so that crank can be evaluated. The "parting line" on a cast crank usually looks like a sharp-ish rough edge and forged crank is wider and smoother. Usually easy to tell by seeing it. As for the 318 cranks, I personally never saw an LA 318 with a forged crank, but some of the older A 318 polysphere headed engines did have it forged. If you have a forged A 318 crank, it's just as good/strong as the 340 forging.
