I'm doing (for about 18 months now
) similar, VL Commodore (RB30E) 6 engine management into a V6 75.
Things to consider are the accuracy of any machined parts that locate the crank angle sensor and the direction of rotation of the crank angle sensor. The V6 Alfa distributor turns the opposite way to the RB30 distributor. A dummied test showed that it would still fire the injectors, but the old RB30 ecu isn't sequential (batch fire) and the CAS sync cylinder slots aren't of different lengths like they are in the multi coil engines. If the CAS is mounted in front of the exhaust cam, sweet! But make sure its accurately mounted otherwise you will kill bearings.
As for driving 8 coils, use 2, 4 channel ignitors. It's just not worth the dollars to find out that 1 or more channels eventually die. Nissan used 'dumb' ignitors, so either 2 factory ignitors or a pair of after market 1s will work fine.
Obviously the original air flow meter is required. Well, at least until you get it running and then do the work to go a larger AFM.
Find out if the TPS is a variable voltage 1 or a switch type. If it's a switch type (like the RB30 1 is), the AFM signal is used for the transient throttle enrichment ('accelerator pump'), so keeping the AFM a similar distance from the throttle body (for NA) or turbo, would be a good idea.
If it is variable voltage (I know the both the 128kb and 256kb versions of the RB20DET ecu are variable voltage), then using that sensor is an obvious must as the Alfa/Motronic system is just a closed throttle switch.
Obviously the use of the Nissan coolant temperature sensor is preferred.
You wouldn't really be able to adapt the factory knock sensor to the noisy Alfa engine (you could give it a go, but I think it would be waisted effort). What you could do tho, is use the knock fuel and ignition maps as a switchable second map, it's a matter of feeding the right voltage to the knock sensor wire to get the ECU to switch to the knock maps.
Biggest challenge will be getting the speed sensor input into the ECU. Modifying the shape of the factory Alfa electronic speedo sensor signal could/should work, but I'm a fitter and turner, so......
I think it's a great idea (but I'm biased). Just do everything you can to ensure the integrity of the Nissan wiring loom for the CAS, EGO sensor and the AFM to make sure the signal shielding is factory intact.
You'll also need ignition on and cranking signals from the ignition switch.