1st 340 build

The big difference, that I know of, between the 318 and 340 forged cranks is that the 340 is balanced for HD rods while the 318 is balanced for lighter duty rods. The 360 ran HD rods that are interchangable with the 340.

If the piston is not going to be an OEM replacement, it would seem that re-balancing the crank is going to be a good idea. Doing this will let you use whatever piston/rod combination you want.

StrokerScamp makes a good point about the cast crank for a less than 500 HP motor. A good, straight, used one would be best as the metal has been through many heat and cool cycles which improves the temper of the metal.

Here's another point worth considering. The cast crank weighs less than the forged crank. That means if all else is equal, a cast crank should be quicker to accelerate than a forged crank because there is less mass to move.