First, contrary to popular mythology, the engine is not a "modified". It is a 351 Midland. Made at the Midland engine plant along with the 400M and uses the big block transmissions. The 351 Windsor was made at the Windsor plant and uses the small block transmissions.
The Ford Motor Company in the United States produced three different engines of 351 cubic inch (5.8 litre) capacity. These were:
351 Windsor (351W) - part of the Ford 90 degree V engine family
351 Cleveland (351C) - part of the Ford 335 engine family
351 M (351M) - part of the Ford 335 engine family
The 351 Windsor outlasted its two intended successors, and was even featured in Jeep-brand vehicles such as the Wagoneer, and Ski Nautique ski boats. The 351 Cleveland was a high-performance engine by design, with larger intake and exhaust valves than the 351W, and was available in the Mustang from 1971-1973. The 351C found its way into DeTomaso's mid-engined Pantera sports cars in the 1970s, but in the United States, the oil crisis of 1973 effectively ended the 351C's career.
The M in 351M has been a long standing and often heated point of discussion. Some believe it stands for Midland, where the engine blocks were cast, and others believe it meant Modified. No literature from Ford Motor Company has ever come to light that proves any one particular theory. As of now, the M is just a letter