having just had mine cut TO SHORT! but useable. I measured from the rear cup holder bolt edge (centre of rear cup) to the rear of the trans tail shaft (mentioned to them 1to1-1/4" has to come off). simple to me but they shorted it anyways!
this is how they want the measurement.
unbolt driveshaft. put trans yoke in all the way, pull out 3/4". measure from centre of yoke u joint cup to the rear centre of cup (where you bolt the driveshaft to with the half round clamps). that is suppose to be the correct measurement.
hope this helps.
ps they will retube mine if I find it to be an issue no charge