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