I wanted to use cars that were raced during the FIA GT Championship era before 2000.
Like you said, the Lamborghini, Ford, Nissan and Chevrolet were introduced in 2009/10.
The Toyota TS020 GT-ONE was not a GT1, it was only the name of the car. It was in fact an LMP (Le Mans Prototype) but was initially designed for GT1. But it was never a GT1 car.
I considered the Panoz GTR-1
The Lotus Espirit probably would be able to output as much performance than the others.
The Ferrari F40 '92 is just a road car and could possibly never keep up with the likes of the McLaren, Jaguar, Lamborghini, Mercedes, Nissan and Chrysler.
The other cars that were classified as GT1 were the Ferrari 550/575 Maranello, the Lister Storm (it is too heavy to spec in this) and the Porsche 911 GT1. The Ferrari and Porsche are not in GT6 so cannot be part of this series.