Paul,
The Craftsman 101.20140 is a bolt-on replacement. It was made from around 1950 up through about 1965 or 70. The model number of the 101.07403 with QCGB is either 101.27430 (24" between centers) or 101.27440 (36" between centers). I understand that the 101.20145 and 101.201451 can be adapted but I have no details. The latter two fit the 1/2 " bed 12" Commercial machines. Your machine has a 3/8" bed so you might have to start with a 1/8" spacer. The two models built to fit the Atlas 10" might be made to fit, although I've no idea what would be involved other than that you would probably have to make a new change gear bracket (banjo). However, it appears that more of the 101.20140 were made than of the two models for the 10". And more of the 2nd and 3rd variant of the 12" QCGB's were made as they were in production for about 25 years versus about 9 years for the 10" and 7 for the 1st variant 12".
Your machine was probably made in late 1951 or early 1952.
Please post the bed length or the nominal distance between centers of your lathe. Unfortunately, although the Sears catalog number is unique for each bed length, the model number isn't.
You will find the parts manual on the 101.07403 in the Atlas/Craftsman section of Downloads, as well as the installation instructions and parts list of the 101.20140