My 67 400 has an aluminium radiator with two puller fans, switched by a thermo switch in an Edelbrock Performer RPM manifold.

Trouble is the fans switch on when the temp guage is reading around 120 deg F and usually don't switch off until just above 100F, about 20 minutes after the engine is turned off.

(Yes the fans have discharged the battery and I now have a switch in line to manually turn them off.)

When the car is travelling at around 50mph the temp gauge is sitting just below the centre (~175F), so it looks like the gauge is reasonably accurate.

I suspect the thermo switch is faulty, but what to replace it with? It looks like 185F and 195F switch-on is the norm for the thermo switches.

Anyone with experience with similar setup?

I have something similar. I use JEGS universal adjustable thermo switch, with a probe that sits in the radiator fins.

Do they make an adjustable switch that goes to the manifold?

I am not to happy with my setup. I didnt think about the rising coolant temp when you shut the car off. It will drain the battery. So I have to put a manual kill switch inline.

Yes.
My 428 has an aluminium rad with a pair of puller fans in an aluminium shroud, RPM intake and a temp probe in the water crossover. I use a 180 deg switch. Running the fans after the engine is shut off didn't make that much sense to me as the water is not circulating, the probe/switch is in the water in the engine and the air is cooling the water in the rad.

I put in two relays on the rad support powered from the main alternator terminal block through two 30 amp breakers. That gives the fans a full 14.2 volts. The relays are controlled from a 12 volt ignition switch source [only hot when the switch is on]and the thermo switch in the manifold. When the thermo switch grounds [turns on] the relays supply 14.2 volts to the fans, when the key is turned off the relays open and the fans quit. A toggle switch is used if the thermo switch fails.

Thanks for the feedback guys.

NSGRN: Yes there is a kit Derale #16749 that is adjustable 150 to 240 Degrees. It has a 3/8 NPT sensor and a separate electronics/relay box where the adjustment is made. I've seen this on a few websites and interestingly the length of the sensor seems to be different in each photo, so not sure if a suitable replacement.

Bluebird428: Looks very tidy under that hood. Same layout as mine with similar (SPAL?) switch on left and gauge sensor on right... some advice on other websites says it should be the opposite but I haven't found any recommendations from Edelbrock.

I now also know where the washer bottle mounts, missing on mine.

The washer bracket was there when I got the car but I didn't notice it or the lack of a bottle until I needed the washer at the import inspection.
I put the temp probe there as it's closest to the hose, I don't know why I thought that.
The relays are spal I think the switch is, but? The spal relays are nicer than the Bosch ones I used for the lights, horn and Ignition, they're sealed and came with plenty of wire attached.