I have a PW-II with the cover off sitting on my workbench in the shed, but the snow is to deep between here and there, to go look.
I'm pretty sure there is room where the existing control module and relay board are mounted for the retrofit board. Just make sure none of the runs on the new board can touch the chassis or cover.
The wiring appears to be pretty straightforward. Here is a schematic for a PW-II and the drawing on the last page of the PW two board to single board upgrade that dvmartin posted above tells you which wire goes where.
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Once the board has been changed, the charger will work just the same. All the board does is turn the charger on and off. The charge profile is built into the ferroresonant transformer and capacitor pairing.