GURPS Atomic Horror is not explicitly part of the Infinite Worlds setting. Certain elements such as aliens using FTL don't fit it in. Most of the aliens will need to be reimagined as coming from other worlds or coming from mystic realms within the world. Most worlds should be low mana, with possible magic, superpowers & psychic powers. The "lost worlds" on page 60 are probably portals.
Here are some suggested worlds.
- Echo 1945-1963 Atomic Horror can be used as a historical sourcebook
- Atomic Horror Use the book as is but without the aliens and secret history. Just monsters & mad science.
- Nazi '50s AH122. The US & Nazi Germany are engaged in a cold war.
- Nazi Invasion AH123. The Nazis conquered the US.
- Futuristic '50s AH123. Earth has flying saucers. Limit travel to within the solar system, which can be filled with monsters.
- Cold War AH65. No monsters or aliens but plenty of weird science
- Total Nuclear War AH70
- Limited Nuclear War AH70
- US Victory AH70
- Soviet Victory AH70
- Conventional War AH71