How late into 70 were LS 6 Chevelles built?

[DSYenko]

Been digging out some stuff to sell and was looking at a "If i remember correctly" a "D" 70 built 512 4 bolt block that was sold to me as a LS 6 block!It has been decked but the owner swore it was the real deal and seemed honest and wasnt asking the stars for it either.Is April to late for a LS 6 block?
I am going to double check the month before I can say for sure. Thanks