The Atlas instructions for both the 1500/1570 and the 6800 say to cut 1" off of the left end of the lead screw (whether it is 5/8" or 3/4" diameter. If doing a 12", you don't have to cut the lead screw. The instructions for everything except the middle 12" model are in Downloads under ... Accessories.
Also, if you check the various parts lists, you will see that the lead screws for the 10" with either diameter lead screw without and with QCGB are different whereas for the 12" they are the same except for the models with the slip clutch.