The United Airlines livery for the Legendary 707 (FS9) can be used for the 707 Captain (FSX), but you need to make some additional changes to the aircraft.cfg file, and also add a texture.cfg file in the UA livery folder for he 707 (you can copy & paste the texture.cfg file from any other livery folder that uses it into the UA livery folder).

This has been explained in the forum here (https://www.captainsim.org/forum/csf.pl?num=1383898760/17#17). I haven't done it in a long time, so I'm not sure that the BMP files are good enough (or can be used) on their own without being converted into DDS files.

Note. The link to the John Travolta Qantas V-Jet in the post I linked above requires you to login to dropbox to download it (only if you want it). I'm surprised it's still available because I haven't done anything with dropbox in years (and the emai account I used back then no longer exists).