Ok this question has been floating around for a while.
Can a CA18DET be put into my U12/UA (pintara/corsair)?
YES IT CANThe CA18DET's came out in RNU12 Bluebirds from 1987 through to 1990 I belive. So yes, the cars came out with CA18DET's as standard (with AWD gearbox's however) in the form of SSS & SSS-R Bluebirds. There are even diesel bluebirds running around in Japan with LD20II motors

.
So we know it can be done, as there are stock Bluebirds running CA18DET's in AWD configurations.
As far as we know, if someone was to swap their CA20E out of a U12 or a UA they would find the CA18DET bolts right up to the same engine mounts and will even bolt straight upto the stock CA20E manual or auto box.
The problem is, thats about all we really know as no one I know has completed or even undertaken this swap.
Its thought that the engine mounts will be fine as well as the gearbox (with a tougher clutch, strengthend auto). What about driveshats and CV joints? Well if you use a Bluebird front cut, its assumed you can simply swap the parts over. Although we don't know if you'll even have to do that! The stock CA20E shafts might work fine...
So to sum up, it can be done. It has not been done yet (that antrx knows of). There are some members planning this swap.
If anyone has anything to add, please do so.