340 crank

As Jimbo said, 273, 318 and 340 all have the same rod and main journal size. The 360 has a larger main journal, but same rod journals.

Lots of cranks over the years, and steel cranks from 318s used to be pretty easy to find. Some of the early 340 cranks were factory shot peened, or so I have been told. Haven't seen it myself, but a few of the 68 to 70 340s that I have taken apart, did have pretty cranks. :D

The 273 crank has small counter balance weights, visibly smaller.
I agree with Jimbo about the sound, but for me a cast crank and forged crank are easy to spot.
A cast will have very sharp and defined parting lines, the forged crank has a more crude looking parting line. The cast crank will have very sharp and square counter balance weights, and the forged ones will be a little rounder.