Cylinder head temperatures

Yesterday I was running the engine, Chrysler M 318B, in the drive to do the layup for the winter, I use a homeade fake a lake plunger, I ran the boat at 1200-1500 rpms for 15 minutes or more, then let it idle for another 10-15 minutes. I could put my hand on top of the thermostat housing and feel the t-stat working. (it would get warmer and colder). The water flowing out of the exhausts never got warmer than tepid bath water. Both sides were about the same temp once the motor warmed up. Then I attempted to stall the engine by spraying the fogging oil through the carb, no way was that going to stall the motor, so I shut it down to pull the plugs and fog the cylinders. Thats when I noticed that the left cylinder head and valve cover were MUCH warmer than right side.

The right side was barely warm, maybe 100 degrees, the left side was much hotter, I could put my hand on the head and valve cover and leave it there, but it was hot in comparison to the other side. By looking at the condition of the spark plug boots, The left ones were brittle and checked, the right ones were not, I would say that this temperature imbalance has existed for a long time. Looking at the flow diagram for the cooling system, it appears that the right hand bank gets cooler water than the left side(All the water exits the motor through the left side). Does this seem correct? Should I tear into the left side and look for a blocked cooling pa$$age, or am I being paranoid?