Toontown and the Speedway pretty much butt up against each other. I don't think it would be too difficult to restructure the loading area to be next to Toontown. If they wanted to spend less money, they could just put an entrance in Toontown and reroute the queue so that it makes its way back to where the loading area is now. As for Tomorowland, tall trees and shrubs could hide it pretty well, but the truth is, it doesn't really need to be hidden. As mentioned, Matterhorn was once classified as a Tomorrowland attraction at Disneyland, but is now part of Fantasyland. It sits on the edge of both and can obviously be clearly seen from both. As long as the queue for the Speedway starts out in Toontown, it shouldn't be a problem.
And to the poster who thought the idea of a Cars theme would be bad...why? The attraction is already about cars, why not give it its natural tie-in? If it's a part of Toontown or Fantasyland it would fit in perfectly. It's one of those few times it seems to make sense to link it to a Disney or Pixar cartoon. Now, if someone starts going on about making Space Mountain themed to WALL-E, then I'd have a pretty big problem with that...