It was probably me Aaron,
I had to change the TKO 600 one from a long one to a short one, to fit the FE.
I assume the T56 has a very similar input shaft to the TKO?
26 spline is what you want. not sure of the part number off hand but don't forget to order a new input shaft bearing as well.
Have a look on Tremecs website. or maybe Mike Fortes.
The 10 spline would be OK with a toploader.
South Wales Area Rep.
DAX Mk4, 383 Chevy Stroker, Tremec. SOLD
Contemporary CCX 3-4028, 445ci Big Block FORD FE, Old School Now Running and Driving