I had a 455 Olds engine that sat for a Long time . It would not turn with a breaker bar . I found out after removing the rocker arm it would turn over easy . I found that two of the valves were rusted at the seats . I took a block of wood and placed it on the top of all the valves and lightly hit them with a large hammer ( lightly ) You do not want to bend a valve . Then I re-installed the rockers and used that engine . It ran fine .