slant 6 overheating

First off would be what you mean by overheating? Is it boiling over? Are you looking at a perhaps inaccurate factory gauge? Do you have an aftermarket gauge and if so what temp it it showing? Correct sender? Has it always got "hot" or has it just started? Have you made any changes recently?

From there, if it is getting too hot, I'd check the timing, carb adjustment. They play factors in heat generated. Look for the bottom hose sucking flat. Have you drained the coolant lately? Air pocket in the system perhaps? Put a new radiator cap on it of the correct poundage.

Those would be my first thoughts. Other factors, but would be negated above 30-35 mph or so would be the fan/fan clutch.

Guess while I'm free thinking, is the belt still on? Slipping? Waterpump okay?

That should give a start.

Hope that helps.

Cheers