Giant Gorg

Both are adventurous anime. Both have similar feel, in both there are alines who knows the secret of the respective world. Both are old animes, but in both the animation quality is excellent. Both stories start from the west countries and ends up in mysterious islands. There are fillers in Nadia anime please skip them. 

One single continuous story instead of individual episodes. Excellent production values for their respective time with very little recycled content. While both have some deeper underlying messages (Conan - environment, Gorg - politics, peace) both are adventure stories first and foremost with brave boy protagonists conquering incredible odds. Conan is a superhuman, Yuu has a superhuman robot friend. Gorg targets a bit an older audience than Conan.  

Both series follow a young boy (Jim Hawkins in Takarajima and Yuu Tagami in Giant Gorg) as they explore an island looking for unknown treasures. Giant Gorg's Captain is also fairly similar to Long John Silver, especially considering his moral ambiguity. Giant Gorg likely had some inspiration from Robert Louis Stevenson's classic tale. Their similarities are just too great.