The Hitachi was the best choice of the standard carbs. Check if you have a wire coming form the carb like this one in the picture that was on the Sunny motor.
It's an idle jet solenoid and needs power to open, connected to the + on the coil. It shuts off with the motor, cutting fuel to the idle to prevent "run on". with out this connected you will be battling to start the motor. If the wire breaks, as it did with me once you can restart a warm motor with an open throttle, but will die if you loose the revs or try to let it idle.
Definitely go electronic distributor. Just grab a standard one off a Bluebird, I think all the Series 3 models had them, they also had the strongest crankshafts. The distributor bolts straight in and only has the 2 wires to connect to the coil from memory. It would be best to get the distributor "re-graphed" to match the SSS spec cam, but would still be ok in standard form as I don't expect you would be pushing the motor to redline on that often and the SSS cam isn't too far from the Std cam. I fitted a modified and re-graphed Pulsar version to the A14 motor on my old Sunny (it had a bigger cam) and never needed to touch it in over 200,000km. Checking and setting points is a pain.
Dump all the pollution equipment this is what kills the L20B. Worth checking, but I am certain that the registration is based on the chassis age not the motor, so our vans pre-date all those rules and regulations. Have you have seen a 60's muscle car with pollution equipment? So why should our vans be any different. Just hook up the PCV outlet from the rocker cover to your air filter and the lower one from the block under the manifolds to the PCV valve on the inlet manifold and you should be right. Just check the PCV is clogged up, which was often the case.
Also in case you are unsure all Datsun/Nissan motors have been built to suit unleaded petrol since the 70's as the motor was designed for their main market, the unleaded USA, and everyone else world wide got the same, just minor changes to the externals like carb, distributor, alternator etc. Again, I ran the A14 in the Sunny from the day it was rebuilt on E10 unleaded with no problems, just had to fiddle a bit with the timing as pinging under load was a minor issue initially. Yes it will run better on 95 or 98. I used to do a lot of miles and decided the 15-20 cent a litre extra wasn't worth it.
Cheers Stephen