Best results, I have seen, with a flatbelly crossmember like you are describing is to run two poly mounts off of the back of the Tcase with 2 more (one on each side) at the frame ends. Chain the motor mounts and roll. If you feel like spending the extra money then some poly motor mounts aren't a bad idea, just not totally necessary.
Other guys I have seen just weld that backing plate off the back of the Tcase directly to a piece of tube and then just have a poly mount at each end where it joins to the frame. Worked fine, I just feel like a solid mount right there would end up tearing bolts out of the TCase eventually.