My Dad has a T-10 4 speed in his 1970 CST/10 with the stock transmission mounts at the 'housing. The T-10 is just hanging from the bell housing by 4 bolts. Considering how many times that truck has been brought up to RPM and launched by side stepping the clutch and the T10 is still just fine... I'd say that a second crossmember isn't needed at all.
Gary