It is a 351M or a 400. the only way to tell is check the stroke. If the motor turns pull a spark plug and rotate that cylinder all the way down. Take a thin / stiff wire and put it down into the cylinder until it hits the piston. make a mark on the wire. then rotate the piston to TDC and put the wire back. make a mark on the wire measure the distance between the marks. 4'' or 3.5''? 4'' is your 400. Crude but it works.