If its a 400, the harmonic balancer will have a notch taken out of it on the perimeter on one side. If its a complete, smooth ring front to back, all the way around, then I doubt its a 400. If it is, with a 350 harmonic balancer it will have a bad vibration. Does someone here have a 400 balancer pic to show him what it looks like. Im not sure how to describe it.