If I consider that you have the "basic" GTD with granada original setup, 10x15 rear should be considered as a maximum IMHO.
I've had some 295x15 Avon some time ago on 10x15 wheels, but finally downsized to 275.
Altough something smaller will provide a better look, IMHO again, with the wheels more inside the wheel archs (something around 9x15), more "authentic". Your own appreciation and the look you want to achieve will dictate. Check the offset carefully, and make some mockup (what I didn't, and now regret !)
8 inch's wide at the front and 10 inch's at the rear regardless of diameter. That would be the standard sizes for a GTD. Before you decide on diameter be certain you can get a tire that suits you. 15 will be difficult and 17s will have a few good choices.
Lots of discussion on GT40s on this subject. search engine should come up with a lot of info, just be sure to check posting date. This subject has be going around and around for a long time.
These are what 17s look like on my GTD. Tires on the 01 number and gold wheels without numbers are 225.45.17 and 285.35.17. On the 83 pictures I am running 245.40.17 and 315.35.17s. Wheels on both are 8 inches wide in front and 10.5 in the rear. All again are 17 inches in diameter.