340 cast crank harmonic balancer stops on crank- help!

Don't drive it on with a hammer. You might damage the thrust bearing. Get a longer bolt that is the same threads as your balancer bolt, run a nut way up on it, add a thick flat washer, then screw it all the way into the end of the crank and then tighten the nut down to pull she balancer into place on the crank. When it bottoms out, unscrew the nut and then remove the bolt and install the balancer bolt and torque it down.