To be clear, the casting ID # and the 66-427 are meaningless - many 352 blocks, 390 blocks, 410 blocks have those same characteristics. The A scratch on the rear bulkhead USUALLY and normally is the tell on the 428 block. USUALLY.
Usually there is also a 428 cast in the floor of the water jacket. USUALLY.
The date code is also important, it should be before the car was built as mentioned above.
No 428 cast into my cylinders. These things are a real spaghetti pie. It entirely depends on what was lying around in the kitchen before they cooked it.
There is also a change over around '64 or '65 where the engine mount bosses are changed to three bolts instead of two.
That generally coincides with identifying a 352 block since those largely are earlier blocks BUT there ARE truck blocks that don't want to conform and associate themselves with passenger vehicles.
The quirks also aren't just limited to the FE blocks. The timing chain covers and the oil filter adapter can have the earlier casting engineering number with a later date and vise versa.
It keeps the "Concourse Judges" arguing all of the time.
The assembly line workers just wanted their pay checks at the end of the week and let the "management" deal with stuff like that.