318 crank shaft in a 340 ???

I believe all 318s were internal balance ( someone here will correct me if I'm wrong...). Your '73 340 external.
Either way there are multiple 318 cranks. Early forged ones, later on cast with balance for light duty rods, and later yet cast with balance for heavy duty rods. None of them balanced for the heavy 340 pistons...
Stroke and journals all the same, so, yes you could use a 318 crank...But needs balancing.