Can't recall the layout for Triumph 1500 engine (75 onwards?) but thought the inlet and exhausts were opposite sides - probably wrong on that count though. A series definitely have them on the same side and a bit prone to heat transferring. Can mitigate by using thermal wrap on exhaust headers and a better carb heatshield that keeps the heat away from the inlet a bit better.
Also there is Zircotec who do specialist coatings allowing you to keep the heat away from the inlet more effectively. Understand the water-jacketed inlet manifold stabilizes temperature better for cold running and higher temperatures.