Chevy used to put there temp sensor in the head, between the ports.  Always had issues when headers were installed.   I've never owned a Chevy, but worked on plenty of friends hot rods.

It's been covered, you can't double up the existing sensor.  It doesn't work in the correct manor for a fan operation.  You can buy generic temp sensors at like Summit racing.  Something like this:
https://www.summitracing.com/parts/vta-11190-vus
Your local Napa may have a cheaper version.

Run it to a relay to activate your fan(s).  Don't rely on the sensor to carry the load of any fan you use.

I would try to get the sensor up before the thermostat housing.  That way you can monitor the temperature BEFORE the stat opens(or if it hangs up you will know it).   

I have two hot rods with very similar set ups, sensor in the intake to activate relays to run the fans.   Since I run cooler thermostats, I get around a 190* switch to just keep it around that temperature and to head off any overheating issues on warm days.   Your climate will differ and may need to adjust for that.

Larry

I'm no expert, but I did stay at a Holiday Inn Express last night.

You are most likely correct buddy. I haven't fooled with street stuff in many years. What we use on the race engines id a restrictor plate instead of a thermostat so the stuck issue does not exist for us. We are only concerned with cooling as we have no problem making heat at 14 to 1 compression. You are right the sensors need to be separate. I thing the spacer we use would work but that is just one idea.I agree they say 10 degree difference on the location if that matters and it might. If he is using cast iron heads and block it would be more important I think.



I cried because I had no shoes till I met a man who had no class.

Going off 30 year old memory so Wayne is welcome to correct me. You need to plumb the two sensors separately. The temp gauge should be plumbed to the head or the manifold. The temp gauge needs to be in the flow path when the thermostat is both open and closed so you can see an overheat if the thermostat sticks closed.. Some manifolds have an extra port in the passage under the thermostat housing to use and some do not. If it doesn't you can use the spacer Wayne suggested. I have always just put the gauge sensor in the head unless I had a reason not to. I have read there can be an approximately 10 degree difference at the head but I have never measured it myself. The factory put it in the head so I figured good enough. It can be a problem with headers and I remember mounting the gauge sensor in the manifold at least once for that reason. The fan sensor can go in the manifold, head or thermostat housing. The thermostat housing should be fine. Once the thermostat opens the sensor will read the temp and turn on the fan as necessary. If the thermostat doesn't open it doesn't do you any good to have the fan turning anyway. Mid 70s SBC thermostat housings had extra ports for various sensors so one from a 74 or so Chevy car should work depending on the sensor thread. There are aftermarket chrome ones with the port also. Some sensors have a large bulb before the threads that can interfere with water flow in the thermostat housing as it extends far into the housing. One of those needs to go into the manifold or head but I had one that would only work in the head as it was even to long for the manifold port.

As a side note if the sensor is used for electronic fuel injection it's probably better to put it in the manifold as the temp should be more consistent across the engine there.

YMMV as it's been a long time.
Marvin

Yes, thermo housing.. 

I cried because I had no shoes till I met a man who had no class.

"can an electric fan temperature sensor be connected also to an existing aftermarket sensor for a heat gauge ?"
I would guess not. The electric fan temperature sensor is probable digital where it just turns the fan on and off at fixed temperatures. The sensor for a heat gauge is analog where it sends a linear signal proportional to the temperature to the heat gauge. Install the fan sensor separately as Wayne suggested.  And I really don't know anything this stuff so take it for what its worth.


Tony

You mean the thermostat housing ?

Benny, In the racing world we use a riser with sensor ports under the top neck. Any race shop or supplier has them. Water returning from the motor is best temp read at the upper neck.Hope this helps.



I cried because I had no shoes till I met a man who had no class.

can an electric fan temperature sensor be connected also to an existing aftermarket sensor for a heat gauge ? The sensor is currently by #1 cylinder. 
If not...opinion of installing fan sensor in aftermarket intake manifold...or installing in thermostat housing....or,if possible,between #6 and #8 cylinders.Thanks in advance to all.