The firing order for a Ford 351 is 1-3-7-2-6-5-4-8, which differs from that of most other Ford V-8 engines. This same pattern is in the 351W, 351M and 351C, as well as the Ford 400 and later 5.0 engines.
According to Reference.com, Ford Motor Company is the producer of three different 351-inch displacement engines. The 351 Windsor is one of Ford's 90-degree V engines. The 351 Cleveland is essentially a Ford 335 with a larger cylinder bore. The 351M is another of the 335 engine family. Of these three, the 351W has the longest history. In addition to its use in the Ford line of products, it is found in various Jeep vehicles, including the Wagoneer and some ski boats.
The 351C includes larger intake and exhaust valves than the 351W. While this gives it more power, it also increases fuel consumption. The 1973 oil crisis is regarded as responsible for its demise, though it was popular in the 1971 to 1973 Mustang muscle cars, explains How Stuff Works.
The M in Ford's 351M is subject to much debate. Some believe it is a reference to Midland, home to the plant responsible for its production, while others stand by the theory it means modified. Ford literature is silent on the matter.