My recently acquired 55 Merc had the same problem my F100 had. The was frozen.

I tried over a week's worth of soaking with penetrating oil to no avail. Then I attached a chain around the bottom of the distributor and hammered on it with a slide hammer for a few days (realizing that I might break it but not too concerned because I will swap out the Loadamatic with a dual advance later distributor). No go. So I took the manifold and carb off and but a 24 in pipe wrench on the base and finally broke it free. Then I could turn it back and forth with liberal applications of penetrating oil and elbow grease. I had to use large channel lock pliers to turn it counter clockwise. If all else fails, you might want to give this a try if you too have a tendency to buy cars that haven't been run in 20 or 30 years.

Sometimes heat works wonders on a stuck distributor Carl