you need to decode the casting stamp on block. if looking at the motor, its below valley cover behind timing cover on RHS. it will say ie 6g5 that will be 6/7/1975 (if my counting is right). first number is the day, letter is the month, last number is the year. i don't know a lot about how far in advance from the cars build date that the block was supposed to be cast but I think a month or two pre vehilce manufacturer is OK