Near as I can tell, it's around 04/01/72 when they changed over from forged to cast cranks. If you're going to swap in a forged crank in place of a cast crank, I'd rebalance the engine, get a '72-up 318 harmonic balancer (so you don't have to modify your crank pulley), remove the weights on your torque converter (or get a 318 flywheel if your car has a manual trans), and reuse your existing timing cover and water pump.
