Interesting. The radiator was recently restored, and the coolant was replaced. Can the Slant 6 get plugged up internally? The travel of the temp gauge was slow and steady.
Any engine can get clogged. Ever take an old engine apart?
I have not used the "receipe" for awhile, where I flushed the whole system with a diluted acid ( I won't mention what so no one will destruct their stuff), then flushed and neutralized it all. Engine blocks and all can get really clogged up.