Confused by the picture you posted. Where is this? What engine is this? Since your list that you have both a 6 cylinder and a small block 66.
Since casting numbers stand up and away from the main body all you can do is clean the surface or any costing, dirt and grime. Then take pictures using light sources from multiple angles if its not visible with the naked eye. You can then play with the pictures adjusting contrast and amount of lighting. This may help
The surfae your looking at from the picture appears that your may be referring to a stamped set of numbers - those would not be casting number but instead assembler, machining, inspector or rebuilders doing.