+1 on the Tridon thermostat mentioned above. The 170 degree unit works well for me also. Ordered mine from an outfit called "Sparesbox" in Australia, and had it by airmail in a few days. Why don't TRF, Moss, and the other suppliers carry these? When the Tridon thermostat opens, the area of the opening is larger than a stock unit and permits greater coolant flow through the radiator, without the downside effects of running without a thermostat at all. The increased opening area of the Tridon unit was measured and calculated by a friend, who found it to be approximately 30% larger than the opening area of a stock Triumph-size thermostat.
+1 also on the need to restrict the bypass hose opening. It's been a warm-climate Triumph recommendation for many years, and has worked well for me in many TRs and Morgans over the past 30 or 40 years.