You could have left MBJ on Pan Am, but they would have taken you to Miami and you would have changed airlines there to continue to NY.
The only direct MBJ-IDL flights on Thursday 27 March 1958 (according to the 2/58 OAG) were the Avianca L1049 that left at 1600 and arrived IDL at 2135, and the it-says-BOAC Viscount that left at 1345, stopped NAS 1535 to 1605, and arrived IDL 2020. The B377 to LHR left IDL at 1700 daily.