Here is some good info about engine serial numbers and casting numbers. Your block should be in the range of 86-87 according to the serial number pictured.
http://www.enginebuildermag.com/Item/1233/rebuilding_the_international_6973l_powerstroke_engine.aspx
Read the part about cylinder blocks.
http://www.enginebuildermag.com/Item/1233/rebuilding_the_international_6973l_powerstroke_engine.aspx
Read the part about cylinder blocks.