Hot water on a V8 use the intake(or the thermostat housing). Hot water on an inline 6 use the head. Adapting a bigger fitting into the head can be frustrating. I have removed the base fitting from the head(not a lot of fun) only to find it is still too small for some Temp senders. With no way to remove the fitting from the gauge capillary(nice big word for ya to think about) You may find it is time to get a replacement Gauge for the brand new one you have already. Not the most fun you can have.
