They did not run on dirt last time, and they did not run on dirt at the nitro wc either. They ran on sugar and concrete, which looked like clay from a distance. IMHO lobbyism from PL, JC, and AKA will play a major role if 'clay' will be chosen, although it does not make any sense - unless you want to continue to sell slick/bar-type buggy tyres, that is.
As for the gearbox, I heard just yesterday that AE has something similar in the conceptual phase, whether for production or just another stealth car.