I ran a survey several years ago both here and on the Groups.io Atlas group as to he belt sizes as found on the various models of Atlas and Atlas/Craftsman lathes. What I came up with for the Atlas 10F with horizontal countershaft was:
Spindle - 4L410
Motor - 4L390
I don't recommend the link belts other than for what they were actually des8igned or developed for which was for temporary or emergency repairs. The reasons are that (a) the original designer did not recommend running them in reverse. (b) if run long enough, they will wear the pulleys with an odd pattern. And (c) decent quality ones will also cost more than the originals..
I did not ask Clausing what the OEM type numbers were as I considered it desirable to remain on good terms with them, given that they still stock at least some of the original parts for the Atlas machines. And those parts that they no longer stock they have been very good about sending me and others TIF or PDF scans of the original drawings for.