The "best" Mopar rear would be an 8 3/4 to swap in. Uses a drop out center section like an 8 or 9" Ford. Non C-clip axles, uses retainers and adjusters at the ends. Can replace tapered roller bearings with sealed Green bearings if you don't want to mess with adjusters, or if the adjusters you have are shot. New adjusters aren't currently available.
A-bodies had the narrowest housings of the Mopars. On all the A-body 8 3/4's they used the 4" bolt circle, so cut down C-body, cut down truck, or aftermarket axles are needed for the the 4.5" bolt circle.
The drawback is 8 3/4 A-body rears are '67-72 only, and weren't all that common new. Performance models had them, and cars ordered with a towing or HD suspension got them.
The 8 3/4 stayed in production in the other models into '74. Was completely phased out by the '75 model year.
If you are willing to narrow down another housing, sky is the limit. C-body and truck housings are the widest, along with the '71-73 B-body wagons. You'll need to move spring perches, or just replace them with some aftermarket ones.
The housing ends are available new. I can't remember from who right now. Same as a passenger car Dana 60. Probably Moser and/or Strange have them.
For the easiest bolt-in Mopar rear with a 4.5" bolt pattern, it's the 8 1/4 from a '73-76 A-body. Look for a 318 car. Not all of them had them. Some had the weak 7 1/4. But if you found a donor with the 8 1/4 rear it probably had the matching disk brakes and upper control arms needed for that swap.
If you want to cut and mix parts you can make a non C-clip 8 1/4 or 9 1/4. The 8 1/4 wouldn't be worth the effort to do that with. The 9 1/4 can be. It's almost as strong as a Dana 60 but it's lighter. Used under tons of trucks. Use the new 8 3/4 / Dana housing ends with some custom axles. Gets expensive though.
No 9 1/4 were factory made that are narrow enough for an A-body out of the box.
An F-M-J body 8 1/4 is the next closest fitting rear, but it's a little wider. Would be very tight under a '64 Dart.
Easiest Mopar rear to fab disks for probably would be the 8 1/4. Snag some of the back of a 8 1/4 equipped Jeep GC. I think the Liberty too. Grab all the hardware, brackets, etc. Should bolt right up to a 9 1/4 as well.
The early Dakota rear is 5 on 4.5", but the housing is wide. I made some rough measurements and the outer flange to flange width is close to the '73-74 B-body. That's too wide for an A.
Moving outside of Mopar, the Ford 8" from a '60's-early '70's Mustang is a straight forward swap. I suppose the 9" would be too, but the Mustang guys want them. I think an 8" from a Maverick or Granada would be the correct width too. But you would have to measure to be sure.
I'm not sure on the 8.8 Exploder rears. I think they are offset to one side and use two different axles. I've heard stories of swapping the shorter longer tube for a short one off another housing and using the matching axle shaft to make a centered and narrower Explorer rear for swapping, but I haven't looked into it myself. I'm sure there are a couple GRMers who have mess with them though.