Tonym47,
The previously mentioned Atlas 1500 and 1570 (later shown as 6800) fit Atlas 10" lathes.
Atlas built three QCGB's for 12" lathes. The one that you want that was made to fit the 3/8" bed 101.07403 (and 101.07361 through 101.07383) is Craftsman 101.20140. When fitted to a 101.07403, the machine becomes a 101.27430 (12x24), 101.27440 (12x36) or no number assigned if the original was 12x18 or 12x30. The existing lead screw is used with no modification (lead screw modification is required on a 10"). For reference (to avoid buying), the other two 12" boxes are for for 1/2" bed lathes and part numbers are 101.20145 and 101.201451 (Atlas 6801 and 6802). As far as I have been able to determine, Atlas never assigned an Atlas number to the first box and did not list them for sale.
I cannot say with certainty that the two later Atlas or Craftsman GB's will not fit a 101.07403. All that I can tell is that aside from the slip clutch on the third one the only part number I see that is different that might matter is the main housing (the operating levers are also different but that wouldn't matter). So unless someone is willing to let you have one of the later ones to try before buy I wouldn't buy one to find out. I do know that the lead screw for the third model is shorter than for the first two because of the slip clutch. But the early lead screw can be cut off to fit.
Robert D.