Check out this Autoweek story on Lambo Doors. verticaldoors.com says they can make a kit for any car as long as they have 10 orders. Kits cost around $1500 a piece. None of the providers seem to have muscle cars listed.
I seriously doubt you'll find 10 Maverick or Comet guys willing to spend that kind of money on door hinges. I could be wrong, but i kinda doubt it. Personelly I can do a whole lot more to my car with $1500 rather than just to open a door upwards. Cars look better with the doors closed anyway. Good luck either way.
I'll go along with dennis on this one for sure. seems pretty steep to me since you can do the conversion for about 200 bucks.