4h at 75...
you must be jokin
75mph/120kmh is a good topspeed, but... 65mph/105kmh is pretty OK and that's why the factory rates it as a max. speed at 4800 rpm (a little bit more than the max.power at 4500rpm).
They say: permanent speed of 90kmh/56mph is not a problem for the 126, in spite of the small engine...
Also: you can shift gears at 5300-5600rpm when accelerating...
Well, 5500rpm is kind of ceiling or about 75mph/120kmh, but have mercy
Something more, you say 500... All models except 500R (8/39 like in 126 all models incl. 600cc) got final drive 8/41. Imagine the rpm at 75mph/120kmh if it can reach em at all, no matter what the clock says.
"Cooling down period" means that we are tormenting the machine or something inside is going wrong - bad mixture, ignition, oil, belt, thermostat... Climbing mountain road at high gear and low rpm... So what means to stop every 50-100-200km, and how to know when exactly? This is good only for the tin in the engine room, so the rubber gasket of the rear window doesn't convert to a bubble gum
Insulating the engine bay is also a good step, cos in a certain moment the walls are just a heat accumulator...
That's how the whole theme started - the open lid
Emin, the oil pan is air-cooled as well
So no alu-radiators will be worth. Well, here is good to say that is good to make sure that the gap in the bottom of the sump is clean and not filled of oil-dusty mess or smashed... Bigger displacement 3.5-4l aluminum pan is other thing.
Have fun, guys!