This from wiki:
400

The big-block FE engine family was getting rather tired and outdated, and the 385 family could not meet the efficiency requirements of the time. At the same time, the small-block Windsor engines were too small and high-revving for Ford's fullsize car and truck applications. So the company went to work on a new small-block to meet the desired levels of economy while still providing the kind of big-block torque that was needed to move 2+ ton vehicles.

The Ford 400 engine was based on the 351 Cleveland but was produced with a taller deck height of 10.297 inches compared to the 351C's 9.206 inches. This allowed for a longer stroke and retain the 351c's Rod-Stroke ratio. These blocks also share the same oiling route in the block. The 400 also featured larger (Windsor sized 3.00 inch) main-bearing journals and had "square" proportions, with a 4.0 in (102 mm) bore and stroke; it therefore displaced 402 cu in (6.6 L), making it the largest small-block V8 made at that time. It was introduced in model year 1971 with a full half-inch (12.7 mm) longer stroke than the 351 Cleveland, making it the longest-stroke Ford pushrod V8 engine. A long-stroke engine has good low-end torque. This was a good compromise given Ford's requirement for an engine to power heavier mid-size and full-size cars and light trucks. The M-block, as it later became known, was the last pushrod V8 block designed by Ford. The M-block also shares some elements with the Windsor engine family: bore spacing, cylinder head bolt-patterns and crankshaft journal dimensions.[1]

The 400 was seen as a smaller and lighter replacement for the big Ford 385 engines, the 429 and 460, in Ford's big cars. Weighing just 80% of a similar big block, it was originally available in Ford's Custom, Galaxie and LTD lines, and in Mercury's Monterey, Marquis, and Brougham. Later, it would power the Ford Thunderbird, the Lincoln Continental, Mark V, mid-size Fords and Mercurys, and Ford light-duty trucks.

The vast majority of 400 blocks use the same bellhousing bolt pattern as the 385 family big-block to make it compatible with the higher torque-capacity C6 transmission used on the large cars and trucks. There were a small number of 400 block castings that use dual bellhousing patterns for mounting an FMX transmission. These castings are rare. The 400 was modified in 1975 to use unleaded gasoline.

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags