Can you post a picture of it? A 400 is pretty easy to spot, however it's almost impossible to visually distinguish from a 351M. Everything between the two motors is interchangeable except the crank and pistons, since they're the exact same engines, just different strokes. I'm not sure if it's possible to accurately measure the stroke in this case or not - otherwise the part number on the crankshaft would tell. This all assumes that the original valve cover sticker is gone - if not, then this is a no-brainer.