67' 318 crank in a 69' 340

340 is forged crank up to 73. I have heard of a few 72 with cast crank but never actually seen one to verify. 73 cast crank 340 was one year only. Also requires a unique damper with an offset balance. Its clearly marked "use with cast crank only".
The difference in the 340 compression was a piston change. The piston with 4 valve reliefs are the later low comp ones. If my memory serves me right, the change was made in 72.