When you factor in the length of the input shaft, the T56 is not longer than a T5. Here's a photo of a T5 and T56 side by side:
The big issue is not the driveshaft but rather the fact that rear mount point for a T56 is much farther back then either a T5 or TKO600.
I have a Factory Five Roadster with a T56 sitting behind a 390 stroked to 445. The standard FFR 10.375 inch driveshaft was to short to use.
After being on the road for 2 years and several trips to the drag strip, I've had no issues.
Bruce