There's a load of sites out there and leagues still running NR2003 so the best place to look is Google, but the league I run with is at
http://www.justracin.net and Jr's truck league is at
http://www.dmp-trucks.com/
http://www.simracingnetwork.com/ is a very good effort to bring live coverage for races online for all to see. And so on and so on...
We didn't quite get the purer GT cars for NR per say (I mean FiA style GT or petit Le Mans). Officially we had the American Trans Am series, Craftsman trucks, Busch Grand National on top of the original Nascar cup series. Other mods for it of note included team Redlines GTP mod, a dirt track mod, the US pits 80's aerowar mod and 70's mod, the OWR (open wheel racing) mod, a dirt modified's mod and a few other stragglers.
The best write up for Papyrus without having to read an epic is probably the piece here
http://uk.gamespot.com/features/6103365/?sid=6103365