I am 99% certain that you need a new driveshaft. The T-56 is a hella big transmission compared to our puny stock T-5's. If you were able to run a stock length driveshaft with your 3550 TKOII, then you definitely need a new one for the T-56.
Yes, you do need a new driveshaft. Or you could have your stock one modified. Call D&D Performance, they will be able to give you the specs for modifying your stock driveshaft. I would just buy the entire kit from them as it is the closest thing to a direct bolt-on as you can get. Oh and I HIGHLY RECOMMEND the MGW shifter on the T56.