Some things are coming back to me: The 351 Cleveland and 351 Modified are different deck heights. The 400 M debuted first, alongside the already established 351C, until the 351C was cancelled in the mid-70’s. That’s when the 351M was born. Same deck height as a 400M. Shorter stroke crank, but not the same crank as the Cleveland, because the mains are different sizes. Rods are the same between the 351M and 400M. They were a pig emissions engine in their day, but with some careful planning, can be updated and made very strong, with proper piston heights, and tighter clearances, and of course the oiling modifications.