72 + 340 motor

Like sireland said. You need the "Use with cast crack 340 only" If it doesn't say that on the damper its not correct. If the motor is being built you can have in neutral balanced (it doesn't take much, I have a cast crank 340 in the GTS) and run any forged crank 340 balancer. Jegs has a nice SFI one. I think Professional Products has a damper you can add weights too and has the cast crank one.