Where were the rifts ? In GURPS Technomancer the location of the hellstorm is of major importance. It makes a big difference whether the trolls first showed up in DC, or moscow, or in africa. Or everywhere at once. And regardless how tough they are, I would assume that a military with fifties tech could hunt them down relatively easily. If Fighter-Bombers with rockets and MBTs are not enough, there are always nukes and chemical weapons. I would be more afraid of gremlins.
https://en.wikipedia.org/wiki/Gremli...ns_in_aviation