Since you mentioned the Mercedes CLK-GTR '97 and the Porsche 911 GT1, PD could very well add these cars to the GT7, following the same idea that was made with the McLaren F1.
The CLK-GTR car is visually different from the CLK-LM, the latter being a little lower. Likewise, PD could place the CLK-GTR in the Gr.3 category, as well as the Porsche 911 GT1 '96.
The '98 version of the Porsche 911 GT1 could be added to the Gr.2 category.
So would it with Ferrari. PD would add the Ferrari F40 LM/GTE '95 to the Gr.3 category and add the Ferrari F50 GT to the Gr.2 category.
Obviously there would be performance differences between these cars and modern GT3 cars, but the BOP would try to do its job properly, lowering top speed and acceleration. And in this way we will have the fantastic quartet of the 90's at Gr. 2 and Gr. 3 (McLaren, Ferrari, Porsche and Mercedes).