Transmission Cooler Question

If you have a clutch fan, Ima thinking in California, it should be a thermostatic type. I have not had any success running a straight viscous coupler. The T-type will keep your water temp at a constant temp, assuming it is capable of doing that. The viscous alone will not do that.
Consider this;
Running the temp sender divorced from the water, like it appears you are doing, will introduce a false security. Divorced, it sits in a pocket of stagnant water and the brass fittings are huge radiators. It could be reading waaaay low.
The factory sender appears to still be there, and that one is right in the hot water.If you have an auxiliary gauge, it too should be in the hottest part of the intake,just below the thermostat.
If I had to, I would eliminate the slowpoke factory sender and put the aux sender there... even if I had to drill and tap for a larger sender.
You should only need an aux cooler if running a non-stock-318 Convertor (which were about 1800s), and only if the cooling system is not running at a controlled, semi-constant temperature. Once warm, my system with a T-clutch, a 7 blade all-steel high-attack fan, and an ancient factory rad, runs at a constant 206, plus or minus just a tic, as measured by an IR gun. However our hottest summer days are only maybe 98*F, and I run above-hood,fresh air to my carb.
Altitude will make a difference too, as will ignition timing..