Same blocks, same heads, same intakes and exhaust, same everything externally... only difference is the crank and pistons. If you have a 351M, and want a 400 - I strongly suggest that - then swap the 351's crank and pistons for those from a 400
The differences between the 351M and 400 are the crank and piston's.To make a 351 a 400 you need a 400 crank which is a 4.00" stroke compared to a 351 3.50" stroke,and you have to use 400 piston's which have the pin's further up in the piston's.The 351 and 400 both use the same rod length 6.580 that's why the pin's in the 400 piston's have to closer to the ring land's.