This is not exactly accurate, although the 351C arrived in showrooms late in '69 it was in MY 1970 cars. There is no documented case of a MY 1969 regular production vehicle with a factory installed 351C engine. All 1969 produced 351 engines were from the Windsor engine family..... cylinder heads using a canted valve design developed for the 351C, which made its debut in late 1969 Mustangs.
If you're referring to the 427 SOHC (or the Cammer as it often called), it wasn't factory installed in any car, because the intended use of the engine (NASCAR) changed their racing rules to ban the design before production could begin, but to salvage the development costs Ford sold it as a crate motor to drag racing teams, many of whom put them in factory sponsored Mustangs.To bad the Ford hemi didn't make it into Mustangs. It made frame twisting power
To be honest that would kill off the Mustang. That would kill off close to 50% of the sales of the Mustang. Ford needs to sell non V8 Mustangs for CAFE as well as to keep the sales numbers up so for those of us who want V8 Mustangs can get them.