If nothing else, sprinkle the setting with quite a few snake cults, using the "At first, everyone thought they were just another snake cult..." line from the movie as justification.
I have the GURPS one, the Mongoose pocket guide and few old AD&D modules. I also used to be a fan of the marvel comics series, particularly the Savage Sword of Conan.
The GURPS one is great, the mongoose one is very good, and the module ones are pretty sub par on the scale. I think you'll be fine with the GURPS one as a world setting book.
I've found that besides reading the original REH stories some of the story compilations have nifty things like maps and letters providing some hyboria context.