Genuine STOCK cars would be interesting, as you said, just to prove which was toughest. But of course, running all out for 500 miles, doing continuous left turns in a big cup, isn't truly, the toughest type race.
Doing 500 miles in the Rockies or the Baha, or even on Virginia's Skyline Drive would be a more grueling test, trying ALL components... uphill, downhill, lefts, rights...doing it in whatever weather presents itself. ...Now, THAT is testing an all-round test of a car !
Hey, Watkins Glen was a nice course for that, but I don't know if they still run the road courses. Hey there are likely some very rural roads in the UP, that would lend themselves to be a great road race course !
Obviously, big bowl racing ovals are preferred to road courses, since the promoter can gain better crowd control, and get every spectator to pay...and pay dearly !