Unless the block was only casted for 1 year, a casting number only gives what the cubic inches are and what years they were casted. You can go to mortec.com and look up big block casting numbers. Since your looking for a specific year, then you`ll have to do it by suffix code, which will look something like this exampe: ATR.
Or you can find out by the date casting on the bellhousing of the block, the date casting will look like this example: C 4 8
C = March 4 = 4th day of march 8 = 1978 or whichever years the block was casted as revealed by the casting number, if the casting number says your block was casted 1975 to 1980, you`ll know it`s a 1978 block.
As the C indicates the months casted, it`s simple, A = Jan B = Feb Etc.
Here is two links that will help you with the casting numbers as well as the suffix code.
Link for big block casting numbers: http://www.mortec.com/bbc.htm
Link for suffix code as well as location: http://www.nastyz28.com/chevy-engine-code-stampings.php