What is the FIRST production date of any C engine installed in anything. Deduct three weeks and that is the date one would expect to see on the block casting date.
That is how Ford works. They change for no one.
Inquire with MCA. Ask what is the earliest known production date of a 351c Mustang.
The truth lies within that answer presuming the Mustang was the first line to get the C?
The 70 4v C's are D0...casting numbers. That is a 70 MODEL year. DEPENDING on when that model first was assembled, the castings were done earlier.
Is the Cleveland a half year introduction? That would make the only sense on the late casting dates.
Incidentally, the 71 Boss 351 4 bolt block is a modified 2 bolt D0AZ (I think it's AZ) block.
They are very rare and hard to find.
They offer NO benefit to using as a "performance block". The D2 block was intended to be the "performance block" and is the one that was "legal" for NHRA Pro Stock back then.
That's where the history of cracking through the main web into the bore comes from.
10,000rpm launches don't do a body good?