Hey,
I'll be happy to measure the driveshaft for you, HOWEVER:
My T56 is a T56 MAGNUM, which is a new version of the original T56, and not the same length as the original T56. I also have extended rear control arms which would also change the measurement.
When I did my T56 install I had to shorten my driveshaft length, I don't remember by how much, but your measurements would likely be different anyway.
If you're using the original T56 tranny, and your control arms are stock, I THINK you can use your existing shaft5 as is.
If you're going to a 4.10 or 3.73 rear you probably want a balanced driveshaft anyway. If not, I would suggest asking some of the guys with the original T56 and the stock rear end to confirm that the original existing shaft will work.
If I can help with any more information please let me know. YOU'LL LOVE THE T56!