The 534 is the only Super Duty with a 4.20 stroke - the 401 and 477 have a 3.75 stroke. You could remove a spark plug, turn the engine over with the ignition coil wire removed ( so it doesnt start ), turn it over until the piston is at top dead center, insert a long narrow rod ( such as a spare pushrod ) into the plug hole, mark on the rod with a grease pencil how far it extends into the hole at top dead center. Turn the engine over until the piston is at bottom dead center, reinsert the rod, mark it again, measure the distance between the marks. Be sure the rod is clean and plenty long enough.