Israeli Ambassador Shlomo Argov, who was almost totally incapacitated by a would-be assassin's bullet in 1982, died last night in Hadassah Mt. Scopus Hospital at the age of 74. He was Ambassador to Britain when, on June 3, 1982, an Arab shot him in the head, causing him wounds from which he never recovered. The assassination attempt was the main official reason for the Peace on Galilee War that Israel launched two weeks later.