Look on the bell housing flange. On it will be the blocks date code. It will look like this example:
B 5 2
B is the month, 2nd month: Feb.
5 is the date, the 5th
2 is the year. 1972. (this will vary depending on the years the block was casted by it`s casting number)
It can also look like this example:
B 5 72 depending.