[Floats]

40 Deluxe,
Aero engines do not run ‘under constant heavy load’. Most aero engines have a maximum speed of about 2700 rpm and climb rpm @2500 and around 2400 to 2450 at cruise.
Aircraft engine speed is limited to propeller speed which becomes inefficient at higher speeds. At higher engine speeds, a reduction gearbox is required to maintain a prop speed of around 2000 rpm, depending on the propellor design.
Dakota engines and props turn at around 1800rpm in the cruise.
Your observation of ‘constant heavy load’ is mostly prop noise related and not engine loads.
Regards
Chris