Do all GM t-56's have a 1 1/8 input shaft?
 

Well, do they? If not all, which ones do?

The only T-56 that does not have a 1-1/8 x 26 spline shaft is from a 2000 Mustang Cobra R and the 2003-04 Mustang Cobra. Both have the 1-1/16 x 10spline shafts. I beleive the Tremec aftermarket 011 box's also give you the option of either 10 or 26 spline shafts.