What distributor? Does it have vacuum advance?
If it's a street engine with moderate compression, and you have good gas, like 92/93 octane, those heads can safely go to 40-42* of timing. That would help. Before spending a bunch of money, I'd double check your timing, making sure your TDC mark is correct. Retarded timing will affect temps at all RPM levels.
I run 40* on my 427 MR in my Mach 1, 500hp, all factory iron, stock brass radiator, stock shroud with Edelbrock pump, and it's fine on the street. Though at prolonged idle it will start to creep on a hot day. If I don't mix some race gas, it will start to diesel on shutoff if it's a hot day, so I use the clutch to keep that from happening. If it's an automatic, you can shut if off in gear. If I mix some 110, it's not an issue.