I think he just needs to keep the same ring and pinion with the rear they are in now. Just swap out the ring gear carrier (cage). install the current ring gear on the good carrier.
I've never tried to fix a broken 7 1/4. If just replacing carrier you just need to inspect ring & pinion gears for chips or cracking of teeth. I've always just replaced 7 1/4 assemblies with stronger diffs so not to worry about it anymore.