Jeremy I run a John Eales 3.9 in my 1980 +8 and have down for over 20 years. Car has done a load of miles all over Europe and in some extremely warm temps, 40C plus. It used to run very hot in traffic, boiled. In the end I fitted an Ali rad, header tank, kenlowe fan and oil cooler and overheating solved. Water temp will still climb from a norm of 90C to 110/120 but the fan soon brings the temp down. I have a manual override switch for the cooling fan so can preempt "impending hot situations". Oil temp is usually 60/70C but can rise to 90C on extremely hot days.
I would recommend having the water temp checked at the bottom of the rad, my Smiths gauge read 10C high. Water on the windscreen on mine came from the header tank pipe leaking under pressure and spraying (finely) up through bonnet louvres. The kenlowe is fused separately, fuse located next to fuse box by bulkhead. I need 25A, anything lower blows on start
