I don’t know about the GT5s, but the Ford-era Pantera used steel tubes that went through the shifter tunnel.
These went up along the driver side kick panel to the heater valve into the heater matrix.
Those tubes have a lot of complicated bends in them, so they are not easy to do from scratch at home, but I thought I saw that the vendors have prebent tubes.
But your car may be different. It would be a good upgrade to run the heater tubes outside of the cockpit under the car (maybe DeTomaso did it), but the coolant has to come into the heater matrix somehow, and that’s likely where your leak is.
Sometimes the heater valves leak too.
Depending on where you live (I.e. how much you use your heater), a temporary fix would be to just jumper the heater lines at the water pump, and flush the coolant out of the heater system with some compressed air (opening up your heater valve full blast, of course)
Then you could take your time to fix the coolant leak.
While you are in there, install some ballcock shut off valves in the system (engine bay) for racing!
Rocky