Is it possible to rig AJ34 engine computer (4.2) to run AJ27 (4.0)
 

The car is a 2000 XK8 with a healthy 4.0 engine and a dead 5hp24. Instead of rebuilding the 5hp I am contemplating replacement with 6hp which should be a direct bolt-on as far as mechanical aspects are concerned (the 2003+ version came with 6HP). Since the ECM and TCM like to chat over CAN bus, I am wondering if I can swap 2003+ ECM and TCM into a 2000 car. The potential stumbling block would be the ability of 4.2 ECM to run 4.0 engine, and re-wiring the car.

Short answer is not to bother with it.

Long answer is you would more or less need to transfer all the major modules and wiring from a 4.2 X103 into the 4.0 X100...

I mean Instrument Cluster, ABS/DSC components, J-Gate, PCM, and all the wiring associated with it, plus most of the wiring in the engine bay as well. And even then you will have to electronically assume the identity of the donor car in order to be able to reprogram any modules. i.e. when you connect the IDS or WDS it will have to read the VIN of the 4.2 X103 that all the parts came from...

Not worth the effort...