Hi there
A fellow has a 57 6cyl 235 chevy engin he would sell me.
Question How can I be sure it is a 57 235?
The link below will show you pics of the physical location of the casting number.
http://www.langdonsstovebolt.com/tech/casting-numbers/Within this same link, you will find a long list of casting numbers, if the engine turns out to be something other than a 1957 235.
If all of the above does not help, there is an engine serial number STAMPED on the flat boss area, where the distributor enters the block. There is another link available to decipher the engine serial numbers, if needed.