I like the idea of a NASA testbed aircraft and if you're using lift engines, I'd be tempted to go with the conformal tanks as the underwing tanks are going to catch quite a thermal load close to the ground. On the other hand, replace the J52 with a suitably more powerful core and use two shaft-driven liftfans sized to fit that area (say, a flying technology testbed for the X-35/F-35 effort).