340 overheating problem

I see by your avatar photo that you have a fiberglass hood.

Do you have a factory style rubber core support / hood seal installed ?

If not, I would suggest that you install one.

The seal keeps engine heat from being drawn over the top of the radiator core support and back through the radiator.

If you search for a couple of my old post on "My cooling improvement project" , I tried about everything you can think of and in the end the core support / hood seal is what was the most and biggest help of anything.

Herb