I have been told that if the casting number on the left rear of the block ends in 0010 its a four bolt, but I have had a couple blocks over the years that had the 0010 and they were two bolts. The numbers on the pad in front of the right hand head under the alt. will tell what year, what it came from original, if its a 4 or 2 bolt. Any Chevy dealer can break that number down.