The earliest 351C "production parts" have August 1969 casting date codes. But it was a couple of months later in the year before assembled engines started showing-up in cars. Whatever source Gary is quoting is fundamentally correct, the engines weren't installed in anything until around November or December.
The 351C 4V was marketed in magazine ads and other media during the second half of 1969. But if you went to a Ford dealer in September 1969 and ordered a 1970 Mustang with an M code engine, it arrived at the dealership 6 weeks later equipped with a 4 barrel version of a 351W engine. By January 1970 the supply of 351C engines was rolling, just in time for one to be installed in that yellow Pantera that made its public debut in March 1970, at the Monza race track near Milan, Italy; then debuted in America at the New York and Los Angeles Auto Expos in April and May of 1970.
Cleveland engine plant #2 went through a very expensive renovation and expansion in preparation for producing the 351C. Those improvements took a little longer to complete than anticipated, delaying the beginning of production. Ford installed all new machinery, the latest most accurate machinery on the market, and they installed an all new computerized manufacturing system, which had to be programmed, started-up, tested, revised, etc. Typical of any computerized manufacturing system.
The cracking of the block mentioned by Doug was a result of operating the engine, equipped with the stock "partially counterweighted" crankshaft, at engine speeds in the range of 8000 to 10,000 RPM. Partially counterweighted cranks exert a tremendous bending force which impacts the mains at bearings no. 2, 3, and 4.