OK, I'm going to spill the beans about where you can fly the round engines...
The Collings Foundation in Stow Massachusetts runs several military warbirds around for airshows. Everything from T6, Corsair, F4 Phantom (I know not round), B24 Liberator, B25 Mitchel and B17 Flying Fortress. They have a pool of pilots from around the country that fly them in their region. There are only a few that actually travel the entire airshow circuit. They are often looking for pilots.
If you are really interested in getting some round engine time, it is a great place to be, and lots of fun. If your in New England, the B25 is taken, thank you very much.