Before jumping into the well / portal, Dirk says "Don't panic" to himself, a clear Hitchhikers Guide to the Galaxy reference.
The number for Blackwing is 555-424-2424, which is the usual "555" fake area code used in many American TV shows followed by the number "42" repeated (a Hitchhikers' Guide reference).
Mr Priest's pistol is a German Luger, most notably used by Nazi officers in WWII.
Mona tells Friedkin that she is a "Holistic Actress". Mona appears to play a Princess in a commercial that Bart watches in her hotel room in Two Sane Guys Doing Normal Things (2016).
The symbol for Project Moloch is two overlapping circles which stands for the two worlds he lives in.