[Fulcrum]

This is off the top of my head, my reference is at home and I'm not home right now. I'm assuming the engine is in the vehical, even if it isn't, the assumsion should help with location discription. But with the intake manifold removed, and towards what would be the back of the engine (nearest the firewall) look at the "cleaned" flat horizontal surface (usually covered by the intake manifold) on the (left)drivers side.
There you should see a stamped alpha-numeric number. Now, that said, if your engine is like my engine block, that number will not have been stamped very well and it will not be legibal. However, and I'm not sure if this will help, but on the outside casting surface on the right side towards the back, you will find a "casting number". Maybe you can track that, I wouldn't know how.
"IF" your block has a stud sticking out of it along the right side towards the back, your block was cast before 1978, that is a clutch attachment.
"IF" your block does not have that stud, and it requires a two piece oil main seal at the rear main, and it has 302 cast in the lifter valley it was cast between 1978 and roughly 1980.
"IF" it does not have that stud and it requires a two piece oil main seal then it was cast roughly between 1980 and 1982.
And "IF" it has 50 cast in the lifter valley and has "short" lifter bosses, it was cast between roughly 1982 and 1986. I believe all blocks after 1986 had the "tall" lifter bosses.
hope this helps.