Those Marelli distributors are about my least favorite Alfa part. As far as I know, the advance plates are not removable with the distributor in the engine. You can, however, set the points if you remove the distributor from the car, set them and then put the distributor back in. I'm not kidding. You might want to consider replacing it with a Marelliplex pointless distributor w/coil (expensive) or a Bosch distributor (not cheap either). There was a thread a few months ago about replacing the points in a Marelli with a Pertronix optical sensor. I think it required some slight modification but if it works you wouldn't have to pull the distributor again for a long time.
Removing the distributor just to set the points turns what should be a 15 min job into a 1 hour job but I think it's the only way to do it right with these things. Put the engine at TDC on #1 and note where the rotor is pointing, make sure it goes back in the same way. Of course, you'll have to set the timing but you were going to do that anyway. Good luck.