Electric Dvorak is the best Dvorak implementation for the Mac. The basic program is an installer that puts the small (3K or so) necessary resources into your System file. Once that’s done, you can toggle between the standard key mapping and a Dvorak arrangement with a keyboard-command equivalent. Either keyboard can be set as the default startup arrangement. The documentation, which is all electronic, is brief but excellent. A small chart of the Key-Caps layout is provided as well.