This is my favorite classic adventure that I've never played and I have waited 35 years to break it out.  After revisiting  it for an upcoming game I found that that the module holds up pretty well after all this time. I could quibble over some poor monster and trap choices but it's easily fixable.
 
I'd love to hear feedback and suggestions from DMs who have run this within the past 10 years.
 
Here's how I'm planning to fit B4 into Hyperborea:

• The Lost City is Xuthus of the Diamond Desert mentioned in Ghost Ship of the Desert Dunes.
• Cynidiceans are now Xutheans, descendants of the original Hyperborean inhabitants of Xuthus who went underground. They spend most of their waking hours in a lucid lotus dream.
• Evil humanoids are replaced with human mutants. These are the descendants of slaves who accompanied their Xuthean masters underground.
• Gorm, Usmagiros, and Madura are now Apollo, Rel, and Artemis. Their cultists in the pyramid aren't chilling in a club house waiting for recruits to stumble in from the desert, they are adventuring parties seeking a McGuffin. 
• The lower levels of the pyramid are 42 rooms of random Expert Set monsters and it needs a lot of work to be gameable. Thankfully, Philotomy's excellent work on B4 handles about half of this for me