Sir; The quickest way to tell if you have a 406 or a 427 is: Feel with your hand on the block underneth the headers (Either side)and see if the Freeze plugs have an "Octagon shape" sorta like a Giant allen key would tighten it in the block. All 427`s except the very early ones had Screw in Freeze plugs, while all other sizes of the F.E Ford (352-360-390-404-428) had normal stamp in freeze plugs.You won`t find any markings that identify conclusively on the outside of the block.Also look on the drivers side of the block near the Oilpan rail for the "Hump" that is made when the block was cast, this is an
Oil passage that the 427 Sideoilers had.If it is a sideoiler there will be three small allen head plugs corresponding to where the main bearing saddles are. This denotes that the block was drilled for the Side oiler passages while if the engine was a 427 Marine Engine - these plugs will not be there...Hence a Center oiler.If it has no gallery or Hump cast in the block then it is definately not a 427. Hope this helps..Mike