Among those techniques that have proven successful, we recommend installing the casemate to the hull first. Then, with a drop of glue on the bottom of the pedestal, holding the barrel, rotate the 5"/51 guns so that the pedestal is horizontal and insert the gun into the casemate through the open gun port. Rotate the pedestal down and fix the gun to the interior deck. Properly scaled 5"/51s will fit through the slot if rotated.
We also recommend practicing the technique beforehand with dry-fitted parts until you are comfortable with a technique that will work best for you.
Hope this helps!