I have a SP Daytona Coupe with the same engine and it never gets this hot.
As Howard says, from a cooling system design perspective overheating is any extended period of operation above the thermostat temperature. When that happens it means that heat into the system exceeds its capacity. This is undesirable.
To figure out what's going on, you and we need more data.
The fact that you have the same engine in both cars is an advantage and suggests a diagnosis strategy: look for differences. How do the sizes of the two radiators compare?
One difference I think you can count on is that the GT40 cooling system is "more spread out", has more connections and has more ups and downs in the path the coolant follows. This means there are more likely to be errors and that it's harder to bleed all the air out of it; it's possible that your GT40 just needs to be "burped." There are other threads on this forum that go into cooling system bleeding techniques.
Also verify that the GT40 system is connected up properly, and that you don't have any hose collapses on the suction side. There have been other GT40s on the forum that had connections swapped that led to cooling problems. If we knew which GT40 you have the experts for that mfr. could tell you how it's supposed to be connected.
Generally speaking, with so many connections and such length it's much easier to mess up a mid-engine cars cooling system.
Back to taking advantage of the twin engines: make some comparisons under similar conditions. Before you do this make sure both cars have thermostats that are a) working and b) the same temperature rating. (One obvious experiment is to swap thermostats. If now the Daytona overheats, you have your culprit.)
For example, at a constant speed of 50 mph or more, do the cars behave the same? If the GT40 misbehaves at speeds where there is a constant and large supply of cooling air but the Daytona doesn't, the difference is probably with air flow, radiator capacity and/or air trapped in the cooling system.
What about constant or stop-and go-below 25? If the GT40 misbehaves only when there is a shortage of cooling air, look at fan size and whether all fans are working when they're supposed to.
>>> the fans have to kick in after only 20 minutes of driving in 80 degree weathe
At what speed and what kind of driving? Any hills?
Edit: I see from another post you have a CAV: there are several CAV owner/experts and vendors here. I'm sure they will chime in.
