318 to 340??

Then either one of two things. Either your machine shop doesn't know a cast crank, or you ran the wrong stuff. No 340 ever came with a cast internally balanced assembly. Cast was external, steel was internal.
Ok. LOL... the guy built indy 500 engines and didn't know a cast from forged crank. I guess I ran the wrong converter, and I put that smooth running 340 in a Demon, right in front of the original 318's converter and tranny, then I put it in a '70 truck, right in front of the original 318's converter and drove it from Illinois to Colo, then it went into a '72 Duster that had a neutral converter, till the car was sold. Purred like a kitten..... :)

Sorry, I had to be a little smart …. LOL.