There are countless things you can look at that "might" tell you what a block was. But with the hundreds of variations that were produced- Not to mention the years and years where people worked on them and replaced things(usually wront) - Its almost impossible to visually glance at a block and know what it is.

The only 100% correct way, that works everytime, to determine what a block is- is to write down & Look up the casting numbers

http://www.mortec.com <--has all the decoding stuff for SB & BB chevy blocks & heads. There are also several books available that list these casting numbers, i prefer mortecs site because i'm a cheapskate.

SB chevy block Casting numbers will be visible on a little shelf at the back of the drivers side of the block - You will probably have to clean some crud off to read it if it is installed in a car-

There are also application numbers & code stamped onto a small pad on the front of the engine on the passenger side- right below the head. You may have to move the alternator to read that one- If the block has been rebuilt, this pad could have been machined smooth, and the stampings may no longer be visible.

When you look at those numbers, there may be several years and displacements that use a given casting number. That is where the front pad's code comes in- that will give you the application as well as the block type- Try to get both numbers and that should tell you for sure what the block is and what it came out of.

-Ben