Re the FLM drag link hitting on the steering arm. I have the black metal sleeves that go in the drag link around the other way. The top of the sleeve then creates a little clearance between the drag link and the arm allowing it to move enough for normal steering.
I am interested in seeing the results of those spiral gears. Hope you have
more luck than this guy. I'm happy with center diff using 52/11 atm but i guess you would need a lower ratio for the heavier vehicle.
The flat rear bulkhead plate is good simple idea. Keeps the rear diff in normal orientation but allows the bottom of TVPs to be completely flat. Would make it easy to add a bottom skidplate the entire length of the TVPs.