Unfortunately, I don't think there's enough room in that space. Yeah, the Speedway takes up a lot of room, but it's not that big. Instead, I had proposed an alternative: out in the northwestern corner of the park taking over part of the Rivers of America, like what is happening in Disneyland right now with Star Wars.
The main obstacle to overcome with this plan would be visual intrusion. Being that the speedway's front straightaway runs all the way from Cosmic Ray's to Auntie Gravity's, it would need to have either A) lots of trees or B) a very long show building and/or rockwork structure in its place in order to separate the two lands from one another.
A better plan, also taking into account what @mharrington said, would be to use the land for additions to both Tomorrowland and the existing Fantasyland.