Here the answer for some questions that you asked:
Weight distribution:
As the S85 has an aluminium block, it is only 20kg heavier than the old S50 and S54 cast blocks.
Plus it is one zylinder shorter than the S50/54. (5 in row, 6in row)
Also I didn't use the mechanic servo steering pump, but used an electric one instead, that is mounted in the back of the engine compartment.
So the balance of the car will be better than before
Boot floor:
The boot floor is a big problem, I know. Mine is very fine, but I will give it to my best coachbuilder to reinforce the whole thing.
Also all rubber bearings will be replaced with solid uniball bearings. That reduces oszillations and protects the boot floor.
Electronics:
I use the standard M5 ECU with Alpha-N Mapping by the very best tuner I know (Friend of mine).
All instruments and warning signals will work as they did before.
The M5 motor wiring loom is connected to the original Z3M interior loom.
That's where the magic happens
No complete M5 loom hidden in the car, that would be a mickey mouse job....