Hello, I hope the following assists you.
Before considering the power output alteration I would see what the vehicle drives like when fully converted as a weight difference will obviously affect the performance of the vehicle. With too little power you would need to drive the vehicle harder to obtain good performance which will obviously result in increased fuel consumption. A 350 Transit is obviously geared to carry heavy loads and detuning the engine won't have any affect on the gear ratios, first gear will still be the same as it was designed to set off with a heavy load without destroying the clutch.
To lower the power output depends on if the Transit is a 2000 to 2006 shape or a 2006 onwards shape, if it is the later shape then a 115PS calibration in the PCM will be OK, if it is the earlier model then you would need to see if a specialist diesel firm could alter the calibration rather than Ford as the earlier 135PS is the only 2.4 TDCi, the other TDCi engines were 2.0 and the earlier 2.4 were TDDi.
You could try an aftermarket fuel saving device, most of these now just plug into the vehicle but have varying claims for fuel savings, an example is http://www.ebay.co.uk/itm/Ford-Transit-Nugget-TDCI-Diesel-Economy-Tuning-Chip-Fuel-Saver-Box-Remap-/260566639295?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item3caafb62bf#vi-ilComp
I hope this helps but if I can assist further let me know and I'll reply as soon as I'm able to.