The 568557 or 557 block was made from late 1975 untill 1979.
Like Creech said, be sure to check your casting numbers.
The 557 is good for a mild street motor, look to see if it has all 5 engine mounting pads drilled, as that could be a strong selling point for anyone with a firebird in mind.
The 1971 engine you have should also have 5 mounting pads, you might check them as well to be sure they are drilled and tapped.
Be sure the block has these for your application, firebird mount locations are different from A and B body cars.
Fact is stranger than Fiction