Hmm, I suspect the engine design may be challenging for that! A Mach 6 airframe is not outside the realms of possibility (it has been done before), but an air-breathing Mach 6 capable engine with a reasonable in-service life and sensible time between overhauls? Not so sure.
For Mach 6 cruise the jet efflux needs to exceed Mach 6, so you are looking at either very clever ramjets, scramjets or rockets, unless the US have managed to design and make an engine similar to the Uk Sabre air-breathing rocket engine concept. It will be interesting to see if this ever comes to pass...