Thanks, Adam, due homework before burning +6000USD
I just have realized that, instead of the bottom of the cameras, I should have aligned the viewfinders in order to ensure the lens is perfectly centered in the housing front port opening (assuming proportion between viewfinder and center of the lens mount will not change, that I think is a fair assumption).
The outcome is that by lowering the camera plate in the housing to accommodate the 2 additional milimeters of the D5, all the relevant back controls should be actionable (even live view!)
On top we only need to modify the access to ISO (the back and front wheels can change exposure compensation in modes A and S) so definitively this starts to look good!
carefully looking at this since my D4 got stolen and I have an empty D4 Seacam housing
Taking Adam´s measures as a guideline to resize images of both cameras (all same length, 160mm, with about 1 mm discrepancy on the height -back view- and width -top view-), I would say that chances to keep access to most of the controls are high with a bit of rerouting and provided your housing has some room (as Seacam does).
Seems like movie rec button would be lost (bigger changes are on top buttons close to the shutter) and front dial wheel might need minor work, together with a slight displacement of buttons on the back