Mate, that thermostat is totally wrong for a Cleveland V8.
Looking at the Tridon specs - it does not have a bypass/block-off foot.
Without the foot, it will make a Clevo run HOT.
Yes, I'd also get the radiator opened up and sussed out internally - if it's a copper/brass tank sort.
They prolly can't open up an Ally radiator very easily.
BUT
get the correct thermostat, and try that first.
Tridon TT2023 or Dayco DT66A/333-180 Thermostats - should both have the Clevo foot.
Post up a pic, when you get one.
Edit -
running no thermostat in a Clevo, will ALSO make it run hot, cause there is nothing to block the radiator bypass port.