Where the distributor base rides in the bore designed to hold it, the O-RING on the distribuor housing becomes hardened to the point of actually freezing in the bore.
When you come across one frozen this badly (and it is common on other engine series also), you try to soak the area immediately at the valley pan where the distributor housing protrudes. LIQUID WRENCH...PB BLASTER. It may have to soak for several days before being able to break loose (use oil filter wrench gingerly around distributor bowl to try and break loose)(may only get a few degrees of rotation at first-re-soak).
Some may be frozen to the point where breakage is the only alternative. Coat the O-RING (sparingly) with anti-seize when setting the new one in.
You can find a replacement salvage, E-BAY or rebuilt.
