Le Sabre is already the most junior model to carry a 9 3/8s differential, so you're not likely going to find a compatible rear that is narrower.
9 3/8s changed significantly across the various generations and no one makes gear sets for any of applications, although i believe you should still be able to get bearings and seals. 9 3/8s rears are pretty hard to kill as long as you keep lube in them.
i'm more familiar with the 71-76 cars ( which are larger than yours ). we have the option, in the small block powered Le Sabres, of an 8 1/2" rear ends configured for the senior cars. into which, any standard Chevy 8 1/2" gearset will fit. i don't know if something like this is available for you or not.
you would be looking for assemblies under any of the other GM Senior cars from 1961-64. Buick models in B-body ( or larger ) would include Invicta, Electra and Wildcat. something like an Olds 88 / 98 or Pontiac Bonneville would probably also work if you took the whole rear assembly.
1965 saw a design shift from the X-frame to a perimeter frame, i'm not sure how that would have affected control arm and spring pocket locations on the rear axle. 1965+ axle assemblies may or may not be applicable for you.
if you get a wild hair and decide to start welding new attachment points onto the axle you can probably make quite a few things work.
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