Keep the 8 3/4, you’d be creating a lot of work for zero benefit and added complexity. It's plenty stout. As mentioned already, factory Hemi cars had them. Dealers don't want people coming back to fix stuff under warranty so if there was any question about the strength of an 8 3/4" Chrysler would have put Dana 60's under anything that had a big block in it.
Personally, I've never heard of anyone doing this particular swap. First, if it's from a 4x4 it’s usually in the front so the driveshaft offset would be in the wrong spot. As with any custom rear end you’d most likely be cutting the original tubes off to make it work under something it was never designed to be used in. At that point you’d be in for housing ends, custom axles and brakes with the proper axle bolt pattern, u joints, etc. Even a Ford 9" would be easier and stronger than a Dana 44.
IMO, your idea to use rear coil overs would likely be a fairly big hassle as well. Leaf springs can be custom made to any spec you want, no need to reinvent the wheel. Why do you think you want/need them? One major thing to consider is how you would mount them. Besides the flats on the axle tube, a true coil spring needs some sort of upper cup/retainer to locate them between the axle and chassis. That's definitely not happening without major chassis surgery/fabrication that needs some solid engineering behind it. Probably would need a trailing arm/panhard bar as well like a GM car. Kinda need to have some pretty good ideas and skills to tackle that job. Unless you're just thinking about those regular shocks with coils wrapped around them? Those have never made much any sense to me.
Honestly, unless you are drag racing a heavy car with a 4 speed that launches at 7,000 rpm, a properly built and maintained 8 3/4" rear will outlast you for regular street duty and most “spirited” driving. if you're that worried about it, back brace it and never think about it again.
Regarding suspension ideas, first thing I would suggest is to stiffen the body with frame connectors. If you do anything, do that and do it before anything else. It will improve overall rigidity and helps to reduce that famous Chrysler body roll.
To provide a real-world example, my ‘68 Coronet has some decent bolt on suspension upgrades; besides fully welded frame connectors we got 1.0” torsion bars, tubular upper control arms (that provide more caster than stock parts), 1” (bolt-on) sway bar, Borgeson quicker ratio steering box (more of a modern car feel than the original numb Mopar P.S. box), new heavy duty leaf springs, spring hangers and shackles. This is all riding on garden variety BFG radials and nuthin' fancy disc brakes.
Just this week I ordered a set of QA1 single adjustable shocks and am waiting on delivery.
Good shocks round out any suspension package by controlling body jounce and rebound inertia. Based on reviews people mostly have good things to say about these things. They ain't cheap so hopefully they live up to the hype and make some sort of noticeable difference. Honestly, just about anything would be an improvement over the current Mopar blue pogo sticks in the rear and KYBs in front.
It’s a basic package but it handles predictably and tracks well. It’s not your look-at me fender flared, ultra low, rubber band tire, Pro Touring/G Machine/road race/autocross bling mobile, just a fun, mostly stock style cruiser which gets driven often in lots of different situations.
And let’s face it, this wasn’t a dirt cheap deal by any means but its nowhere near the top of the suspension mod food chain and should be within reach for most people that mess with these cars.
Lots of options out there to make your junk handle even in a mild sense.