mtflat
Flatheads Forever
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No way to tell bear, without the sticker off the oem valve cover. Externally the 351M and 400 are identical. Only the stroke is different.
351M Bore 4" x 3.50" stroke
400 Bore 4" x 4" stroke
351M Bore 4" x 3.50" stroke
400 Bore 4" x 4" stroke