After A/C Add On Engine Overheats Really Bad

Sounds like there's air trapped in the cooling system. If so, it'll create steam, and that will make the engine run HOT.
Fan shroud is a MUST. Nothing wrong with a flex fan. I have a 11.5-1 compression 440 that runs so cool, if I run it without the thermostat, the gauge won't move from the "cold" pin, and it has a stainless flex fan.

Good Luck!

George