I've run into this a few times. Whats holding the Distributor in is CRUD. Usually old varnish deposits. What I usually do is keep twisting it back and forth while lifting up on the distributor. You can also use a small pry bar/big screwdriver at the base to GENTLY help pry it up, alternating from side to side. I also usually spray liberal amounts of penetrating oil down the housing (plan on changing the oil afterwards).

Just keep working it and it will eventually come out (I have sometimes had to work more than an hour on some of them, but they do come out).

One last thought. If you get really desperate, get a shop manual and study how to dissamble the distributor, it is possible to change the points cam without removing the distributor. Beleive me though, it easier to do with the distributor out, and chances are the bushings are worn to the point where they need replaced (which does require the removal of the distributor).