I suppose it is a bit late to come with suggestions now, but just for the sake of getting the information out I will write down one other idea.
We have been driving around sporadically in a 41 Chevy sedan with a 216 for many years. For a 25 year period or so the engine had a strong tendency on hot summer days to overheat quickly (according to the temp gauge.) After trying about all of the suggestions that floated by in the above posts we noticed in the owner's manual that the 216's in that period were expected to carry a 160 degree thermostat. Over the course of the years some long-forgotten FLAPS had sold us a (standard) 180 degree thermostat, and apparently the dash-mounted temp gauge thinks that anything around or over 180 is almost in the red zone.
We exchanged it with a 160 and now the gauge rests right where we like to see it. Others may have more to say on the subject, but this was our experience.
My truck has the 180 and seems to like it just fine.
