There's not any physics in NR2003 that would make for a good driving legends car... from what I understand though, there's a legends mod for rFactor.
That could be said about most mods for nr03 though. People aren't going to play your upcoming DirtLM mod because of realistic physics, because well there aren't DLM physics in nr03, they are going to play with it because they love DLM cars. Same for just about every mod out there.
Although one Idea I've never really seen kicked around with the Legends cars is scaling them up. After-all Legend cars are just 5/8 scale full fendered versions of the Modifieds that ran during the early years of NASCAR. 2 sedan styles, 2 coupe styles, with or without the fenders/running boards (depending on what looks better full scale).