My 310 also ran hot during summer months, YES UK Summer!
First; The history and bills that came with the MH showed two recent major services to the cooling system within the previous two years, including fan clutch, water pump, thermostat, radiator and hoses.
My initial approach was the radiator cap. Sure signs of weeping here as every time I stopped and parked up the overflow was dripping. New cap stopped that.
Second, and probably not the answer in the USA, I removed the AirCon Condenser. that did make a difference.
It would appear that the dash AC has never worked since a major rebuild in 1987 - the controls were never connected! Sooooo out it came. Now the temp struggles to reach quarter of the gauge.
This left me with two dilemmas. Is the thermostat stuck open or was it always the condenser that stopped air flow through the rad. I favour the 'stat, as it takes ages to reach any sensible working temperature. - Is the 'stat and gasket the same as any other engine? Over here I am struggling to find a replacement.
My next project when I get into my new new workshop (Concrete floor and dry!!!) is to remove the cowl and viscose fan from behind the radiator. I have already fitted an electric fan in front where the condenser was, and that can push more air than the viscose ever did, however I am wondering about engine bay cooling now, after all the viscose fan ALWAYS was pushing air over the engine no matter what temperature. I would like to fit the cooling vents found on the later Classics if they are still available. Any thoughts Chaps?
My last concern is the engine oil cooler. It is the big heat exchanger in the front, whilst the auto box cooler is in the bottom of the rad - is that correct? I would have thought it should be the other way round.