Take your yoke, push it all the way into the transmission with evenything else in the position you are going to be driving.
Place a scale next to it and pull it out 1 inch. Measure between the center of the U joint to the center of the U joint.
Tell the man you want 'X' and you measured center of U joint to center of U joint.
I just had one done after I swaped transmissions and that was how they said to measure it.
Hope this helps.
