One give-away are small journal blocks drilled for 4-bolt mains. These were the blocks that were set aside for Ram Air and HO motors. This was done at the casting foundry after the blocks had been cast and inspected. (Only the "best" ones were set aside.)
As the produciton run neared its end, these blocks were used up for the more common 4-Bbl. applications. Most common (that I have found) are the YS & WT bolcks with late '69 & late '70 production casting dates. Neither of these motors have 4-bolt main caps, but many were drilled for them, attesting to their higher than average quality as deemed by the foundry.
Though you don't need the 4-bolt mains, these blocks were considered "better" at the time they were produced.
