Lee, if you look at the first statement in my post, I listed drivers who came from those series as the obvious ones, just to get them out of the way.
As far as IROC cars, I have heard that they are closer to a Busch Grand National car due to their relatively low horsepower, but the IROC cars have significantly more downforce than a Winston Cup or BGN car due to their rear spoiler. Consequently, IROC cars are much easier to drive than a Winston Cup or BGN car. The higher downforce on a IROC car was designed so that NASCAR drivers wouldn't have an advantage over open wheel drivers since NASCAR drivers race cars with 4-5 times less downforce than IRL and CART drivers.