I discovered a conflict between the radiator drain plug and the OE intercooler which prevents it from moving fully into place vertically:
The offending feature is this web:
Grinding away a piece 32 mm x 9 mm fixed the problem:
Here it is installed:
An added note, connect your intercooler fully before reconnecting your coolant hoses and filling the system. The cold pipe coming out of the intercooler and connecting to the charge pipe can't be installed once the coolant hoses are in place. Ideally, get it into position as you install the fan shroud to avoid some cursing.