Cooling fan turns on late, when the heat gauge is at 3/4.

The first big question is the gauge accurate? Is the engine really getting too hot or is the gauge misleading?

The gauge is probably computer driven and can only display the reading provided by the computer based on information it receives from a sensor. Sensors have been known to grow tired...

You have almost overhauled the entire cooling system - there isn't much left.

Overheating is caused by more heat being produced than can be dissipated by the cooling system and possible reasons are insufficient coolant flow through the radiator or insufficient cooling air through the radiator or a combination of both and you seem to have covered these.
When replacing a radiator it is well to have the original rebuilt by a local specialist as they can usually find a suitable core with an extra row of tubes.

It is possible with many engines that when the degassing system becomes blocked bubbles of combustion gas can accumulate around the thermostat and disrupt the proper operation and when a cylinder head gasket begins to fail those gas bubbles can become excessive. If they can't harmlessly rise to the top of the coolant level they can build up and restrict flow and displace coolant and cause hot spots in the cylinder head(s).

Sediment build up in the cylinder block has also been a cause of overheating...

I am a little confused here, you say the temperature gage is in the red but the engine is only at 140-160? Is that Fahrenheit?

Check your cooling fans and cooling system. Drain and flush your system if you have not done so lately. Make sure your radiator is clean.

Sounds like the electric coolant fans are not working properly, sometimes one or both burn out and need replacement have this checked and tested before any further damage occurs to your engine from over heating.
With the hood open put the fan and defroster on high blow to see if this will temporarily help get the fans working until you get it checked.

The car overheats because the fan system is not working. The engine coolant sensor tells the computer to turn on the fan when it gets hot and also sends temperature readings through the computer to the heat gauge. Try turning the AC on if you have it--it should immediately start the cooling fan. If nothing, check the fan fuse and relay. It's also possible that the fan motor is frozen due to bad bearings. Try replacing the coolant temperature sensor to see if that restores the gauge readings. Hope some of this helps!

have you had the coolant changed lately? there may be an air pocket in the cooling system which can lead to a blown head gasket.

I have a 1996 SL1. The cooling fan automatically turns on when you turn the A/C on. If the A/C is off the cooling fan will only come on when your temperature gauge reaches almost 3/4. I've had my car for 13 years and this has always been the case.

Before getting too upset about this, try changing the temperature sender. Can't tell you where it is but it is usually on or near the thermostat housing but some engines have it on the intake manifold or rear of cyl head.

there is no computer to turn the fan on. it works on a thermal switch. so you have a relay to the fan. when the temp sensor gets hot it connects the circuit to the relay and then the fan is on. vary simple. and it works when the car is still hot but the key has been turned off till the switch cools off or some have a timer so it does not run for ever. so good guess is the temp sensor for the fan could be bad if the fan is working.

Numerous things can cause this, plugged rad, faulty thermostat, low coolant, cooling fan not operating, bad cap,and if it's left too long or overheats too much, you could cause major damage like head gasket, head cracking. The later are major expenses, so don't let it get to that. Does it heat up on highway, or in stop & go more? Does turning your heater on, make it cool down a bit? Do you know if your rad has been flushed lately, and thermostat changed?