Do not know the Dexta clutch in detail, so may be just a few trivial things...
1.5 inch sounds like either the length of the pilot bearing or the length of the splines.
For the pilot bearing: look towards the front and check whether the splined holes of the clutch disk (or disks?) and the hole of the pilot bearing are in line and concentric.
For the splines to engage: rotate the transmission input shaft (or the engine crank) through, say, 15 degrees while trying to move front and rear half of tractor together.