The impossible to solve overheating problem

BigBlockMopar said

Any chance you have installed a Marine or Magnum type waterpump?
Magnum engines use a serpentine belt and run the opposite way of a LA waterpump.
The waterflow is still the same in the engine, but the (Magnum) impeller might be inefficient with your v-belt rotation/setup and hardly pump any coolant around now.

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Havent changed the water pump at any point, did compare the impeller blades to an LA water pump online and the blades were the right direction, provided that picture online was accurate.

But like you said, the new V-belt does not have a great contact patch to the water pump.. It seems doubtful that it would slip but it is definitely a possibility, that is one thing that was changed right before this overheating problem.