The "ideal" A350F would encompass the landing-gear, centre wing box and stronger wings of the -1000, in a fuselage one frame longer than the -900.
Based on that, the weights of a A350F could be roughly as follows.
MZFW: 230t (+7 tons over the -1000)
MTOW: 310t (- 9 tons over the -1000, trading TOW for ZFW)
MLAW: 240t (+4 tons over the -1000)
Capacity: 28 main-deck 125/96, 6 + 6 lower-deck 125/96.
That will, all things being equal, allow it to lift 90 tons and take off with around 80 tons of fuel, enough for a 12-hour flight + reserves. That would make it an extremely valuable proposition. It would offer more volume, more range, 20% less fuel burn, reduced maintenance and reduced variable operating costs (landing, enroute charges) over a 777F, but with a maximum payload of around 12 tons less. It would also render the -8F largely irrelevant.
I had guesstimated the OEW at 140t as well.
On the other hand, I think that your are being pessimistic with the MTOW. It doesn't need to be lowered in order to increase the MZFW, other options are available like reinforcing the structure where it needs to be, impacting the OEW though. For example, the 748F has a 13% higher MZFW than the 748 (334t vs 295t) while maintaining the same MTOW (*). A development cost in the range of $2-3 billion points at a significant development which could include an increased MZFW.
In my estimates for this shrunk A351F, I had considered that Airbus would aim at an equivalent max payload as the 772F. That's 102.8 metric tonnes. So I have the following numbers:
Max payload: 102.8t
MLW: 252.8t (based on your numbers this would be +16.8t over the A351 MLW, or +7.1%. For reference (*), the 748F has a 12% greater MLW than the 748).
At max payload, this plane could take 73.2t of fuel. Considering 6.5t fuel remaining at landing, this leaves 66.7t of fuel to burn. With an average fuel burn of 6.7t/h and considering an average speed of M0.82 over the entire flight, this amount of fuel is good for 4672 nm still air. A 319t MTOW would allow 3t more of fuel and a range of 4880 nm.
By comparison, the 772F is said to carry 102.8t over 4880nm (**). (**) also gives us the MTOW and MZFW of the 772F. So the fuel burn for carrying 102.8t over 4880 nm still air is:
This leaves 99.35t for fuel at MTOW and at max payload. Considering 8t of fuel at landing, the 772F would thus burn 91.35t for such a flight.
So for the same max payload 102.8t flown over 4900 nm, my much assumed 319t shrunk A351F would burn 69.7t vs 91.35t for the 772F. This would be 23.7% less than the 772F. This is great provided that the 140t OEW and the average fuel burn of 6.7t/h are correct assumptions. Note that I'm assuming that at any given weight during flight, a shrunk A351 would burn slightly fuel than the baseline A351 given that the shrunk fuselage would generate less friction drag. I just have no idea of by how much this would reduce the fuel burn.
Note that (**) is also listing the changes in the 777F compared to the 777-200LR.