Checked the news archives and confirmed that PanAm was back in Singapore by 1979.
As for cargo flights, attached herewith is a photo of a PanAm Cargo 747 landing at Paya Lebar Airport in Singapore, date unknown... but with the Garuda DC-9 in the foreground in its old livery, am guessing its about mid/late 1970s.
By the way, anyone know the registration of the first Pan Am Boeing 747 to land at Paya Lebar Airport on 2nd July 1971?