The easiest way I've found to indentify a 400 small block is by looking at the side of the block. The early 400's have 3 frost plugs on each side. The later 400's have 2 frost plugs and in between them the block has a raised boss for a frost plug but its not machined for a plug. I have also found if the block has 3 frost plugs on each side it's most likely a 4 bolt main. I'm not saying this is 100% correct but this has been what I've learned over the years. Don