Question: Is it possible to install the CHT sensor into the EGR hole of the 1800cc head? The threads aren't the same but I feel I could "adjust" it somewhat.

NO NO NO!!! They are NOT EGR ports. They are air injection ports for the smog pump.

EGR is exhaust gas recirculation. EGR is exhaust gas being added to the intake mixture to lower combustion temps to stop the formation of NOx.

Air injection is used to add air to the exhaust ports to complete the burn to lower HC.

If you stick your #3 temp sensor in the exhaust gas flow, you WILL melt it. It is aluminum!

Get a 10x1.0 tap and a drill bit and enlarge the tin screw hole forward of #3 intake port.

PLUG the air injection ports.