blocks need to be properly cleaned, all oil gallery plugs need to be removed and brushed through. Oil restrictors need drilling out too and this is something that is rarely done. I watched my own engine block be cleaned, it took a total of 3 hours work before it was satisfactory.
45% of rebuilt engines prematurely fail due to contamination from a 'dirty' build. Clean it properly, keep it clean and your 1/2 way there to a sucessfull rebuild.