From what I've seen INS reminds me of Loran C, maybe even a bit more primitive.  Doppler on the 707 is very easy to use and works great for me. In real life when was INS used rather than doppler on the 707? Look as though both systems accomplish the same goal.
Although all this is so outdated, it's fun to have the opportunity to use all this from a historical perspective. Makes me appreciate modern tech. even more. Old school nav. systems are fun but so time consuming and labor intensive