You sure there was a 2002 Viper GTS in GT1?Lined up the car models in my garage(again), that were in GT1, currently in GTS:
NSX Type R
WRX STi Type R
22B(similar to Rally version)
If we get the classic tracks list, I'd run these "original" GT cars around those circuits in Custom Race. As they are, feels very nostalgic driving these at Laguna Seca.