Since I posted this, I have been advised (on another forum) of the layout on the Halifax.
As you say, in the Halifax WOP compartment, the transmitter and the receiver are side by side, not stacked as per the Lancaster, so there isn't any room for the Fishpond Unit to be installed on the left.
However, the "table" has a cut out on the left side and the Fishpond Unit (and presumably the Monica Unit) was "dropped into" the cutout section (ie directly in front of the receiver).
Hope that clarifies
Researching and documenting the History of No. 35 Squadron