340 X heads ?

What is the year stamp on the block? I am betting it is 70's left over stock or it is not original to the car.
didnt know how to read the dates back then,..i was 2nd owner,had 42,000 miles on it when i got it...never went in to it ether but seen the 340X cast in the block with my own eyes an the X marks on the heads....doubt the lady that i got it from ever hurt it ether!