"No, I meant I will start with the 79 USAC, and then work backwards. As far as I'm aware, CART from 79 onwards is already available"
As most all USAC/CART/CCWS/IRL cars have had unique liveries for every single car, usually even for cars of the same team, it would really be nice (and much more realistic) to have these liveries available for GP3/GP3-2k, rather than pairs of cars with the same liveries, as we have now. (And, of course, we could always use better quality cars and tracks.)
The website that I mentioned above is pretty much the only one where you will find any reasonable quality color photographs of USAC/CART champ cars from the early eighties and earlier. If you are interested in black and white photographs from any years or color photographs from later years than 1979, I could direct you to a few more websites.