The 6HP26 was never designed to physically mate to the M62 engine nor the NV225 transfer box and the electronic/control integration issues would be insurmountable, in my opinion. It would be far easier & cheaper to sell your car and buy a later 6-speed model than to replace the 5-speed box with a 6-speed unit.
Although the limitations of the infamous GM 5L40-E transmission used in the TD6 are very well known, the ZF 5HP24 used in your V8 is less problematic and issues are mainly confined to two areas – the B-clutch hub/C-clutch drum axial bearing failure and the torque converter lock-up clutch vibration.
Regarding its torque capacity, although the 5HP24 was originally specified for 420Nm, and the L322 M62 engine produces 440Nm, the same transmission with identical internal running gear was later used for the 4.6is BMW X5 which produced 480Nm and the 4.8 litre Alpina E39 B10 V8S & E52 Z8 which produced 510Nm. These higher torque applications used a 280mm diameter torque converter rather than the 260mm unit.
By the way, the 6-speed isn’t without its issues either (solenoids, stator bush wear, etc.)
Phil
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