Take a look at Kenlowe fans in U.K. They are high quality and they supplied the fans on the original GT40. I simply use a thermo switch in the water jacket of the 302 inlet manifold to a relay with a manual override switch on the dash. It's a twin fan set up that I just run on the high speed option. This solved all my marginal cooling issues.
You can put a thermo switch in the rad but I prefer to know it's working off the actual engine temp.
Also check the thermo switch. I found two switches from a well known make way out of spec. I have the auto switched fans coming on at 100degC and off at 95degC although I usually use the manual override a bit sooner when the temps are climbing. No need for a very high pressure rad cap. 13psi is fine. This, together with anti freeze in the coolant, gives a boiling point well above 100degC. From memory about 120degC but I need to check my notes and calculation for the precise number.
Hope this helps.
Is there a reason to abandon the stock fan control scheme? Most of the SLCs that are running LS engines are using some version of that.
My car uses a common mod that ties both fan outputs (one from the GM controller and the other from the AC) to drive both fans when either output requests a fan. It's easy to make that mod, and in practice, there is very little hysteresis, at least in my car.
If I were building my car again with the stock controller and Infinitybox, I'd do the same thing.
Why are you looking at manifold temp to control the fans?
Surely the correct temp to control is the outlet from from the radiator.
If the radiator outlet temp goes up the fans need to run to cool the radiator fluid, once below the set temp the fans turn off. When the fluid is cooler than the set temp the engine temperature will regulate by the thermostat opening or restricting the flow.
I have a Ford radiator fan switch (about 19 mm thread) in a hole in the radiator next to the outlet. From there it goes through two relays and one energises each fan. 10 years on system works well. Fans cycle in slow or stationary times.
In the UK there were 3 Ford switches each with a different turn on and turn off point which were colour coded.
I also have a dash mounted override switch, when doing France trips and approaching the Toll Booths and queues I hit the override to keep things as cool as possible, mainly for me as the heat soak into the cockpit increases dramatically if I wait for the thermostat to kick in and turn the fans on. Block temp gauge shows 82 with manual override and it cycles between 85 and 92 on the thermostat.
In a Corvette or similar the manifold and radiator are the same thing with not much distance between them, but with mid engined cars we have about 7 foot
of pipe in each direction rad outlet and engine manifold can be way different temperatures!
If the car is morning above about 20 mph it will not make any difference if you have the fan running as there is already enough air flow to remove heat.
No aircon so no way to add to that side of the discussion.