There is that possibility.
It would involve trawling through European eBay sites to obtain a transmission drive train, ecu and shafts etc and the cost to get it all transported to the UK. But again.
Iirc the hubs and their respective spline holes are different between the 166 and 156 as is the length track of the shafts. The bottom wishbone ball joint position is different to so even if you could fit the hub carriers from a166, the geometry would make the car virtually undrivable.
In theory though and with precise cutting, balancing , sleeving and welding, you could use the 166 5 speed jtd auto box, inner drive shafts halves, joined to the 156 q system outer halves so using existing hubs, cv joints and existing suspension components.
The other complications are the ecu matching and loom with sensor patching.
You have to remember that the q system auto box ecu is integrated with throttle control, emission control and abs speed sensors and more importantly, to the engine ecu.
There is a world of difference between rev limitations between a V6 and a JTD. It's no good having rev limits, change up, change down response limits designed for a 5000 rpm limit and trying to use it in a 7000 rpm engine with a high power and torque band. So...
It would involve a custom ecu parameter set up assuming you can get the box and engine ecu to communicate initially together.
Tempting as it sounds to have an extra gear, the unforseen costs and issues would put me off right away from even contemplating it.
That's why I suggested the Gta selespeed system that is already matched to the engine characteristics, hub carriers etc., even better if you can buy a rear ended Gta selespeed and just swap all the front end engine, gearbox engine loom, subframe, suspension, brakes and then you can forget the hassle of the auto box ecu Integration.