The distributor on my 312 cannot be rotated. Given its location at the back of the engine, it is difficult to get a grip on it or find an angle where I can tap on it. I suppose that if I remove the intake manifold, there is a better chance, perhaps with a strap wrench.
I am hoping that someone else has dealt successfully with this issue.
I suppose by now you received some advice but just in case, I had this problem years ago on a 1952 Mercury flathead. Fortunately the distributor was in front. I solved it by applications of penetrating oil multiple times at the distributor base for over a week. The oil is better absorbed when the engine is warmed up but not too hot as to be a hazard. You’ll want to apply the oil liberally as some will “burn off” during application. I suspect that in my case the timing had not been set for a long time so the distributor somewhat froze in place.
Thank you. As I currently have the carburetor off, I now have a little better access. My mechanic brother suggested using a hair dryer to warm the area. I will try that together with the penetrating solution.