I see what Hawaii's done there - definitely the way to go. Get your preferred tires and casters rated for the weight, a few bits and bobs welded and drilled to fit/bolt securely and make a proper dolly. Use a floor jack to raise it up on the dolly, attach and done.
If the day ever gets here that I work on it again this is probably the way I will build them...hopefully get large enough casters where pushing it outside might be possible....