[Piotrsko]

I would suggest that if you are hauling heavy things, a modern transmission with more lower closer ratios would be a good value as opposed to odd differential gearsets.
Don't know if you can do tranny swaps like here in US without making adapters.
Accelerating heavy masses is where the better efficiency helps, once in steady state aero becomes important
Drove a close ratio tranny with a close ratio brownie and except for rowing the gear sticks made heavy stuff better to run on the streets but you never have a perfect gear.

If you're shaving the guides, doesn't that include some porting?