A great gear to use – but I’m not sure they ever installed any into an FMJ body.
IF they did, it would be pretty hard to find (extremely rare).
http://www.ebay.com/itm/CHRYSLER-7-25-AXLE-RING-PINION-3-23-RATIO-C7-25-323-/182383515490?hash=item2a76e79762:g:Iz8AAOSwo4pYSu3g&vxp=mtr
http://www.ebay.com/itm/Yukon-Gear-Axle-YG-C7-25-321-Ring-And-Pinion-Gear-Set-/291730302848?hash=item43ec7b2780:g:5ecAAOSwbwlXBDoX&vxp=mtr
These were found on eBay.
I would also check with FABO and FBBO (For A Bodies Only and B bodies) for sale/parts wanted - for it would be a lot more likely to find 3.23 (or a 3.21) in one of those cars.
I am fairly sure – but not 100% sure, the ring and pinion gear set from ’60 all the way up to when Dakota’s quit using the 7¼” differential (‘89/90 or so) will fit OK into a FMJ.
The differential carrier and axles might not fit (more limited upon fitting into an FMJ).
Recommendation number 1: You might want to consider adding limited slip at same time.
Recommendation number 2: Replace all of the differential bearings when you have it apart - and use quality parts (don't skimp on bearings). Some differential shops might not do the repairs without replacing all of the bearings.
Recommendation number 3: if there is a possibility, you might be using/making more power than a 318 2-bbl will produce, sometime during the cars life, you might want to consider finding an 8¼” differential - first. Parts are more available and less likely to leave its guts behind the car the first time you punch it.
If no plans more than a stock /6 or 318 2-bbl, then your plan of gears may be all you need (and a corresponding speedometer gear replacement to keep speedometer/odometer correct.
BudW