Rear planetary interchange?

Got the unit torn down (1985 build A999) and found both planetarys in not so great condition, fair amount of side-side and rocking play. Given the mileage and hauling around a huge van I'm not really surpised. Got a line on a new front unit for a decent price, rear one I am not having as much luck locating or finding out if the newer part numbers will interchange/superceed the older ones. Original part number for 85 being 3681988.

I know chrysler changed the sprag and drum for 1988 but will the newer unit fit the older drum ? Cant think of why they would go through all the trouble of changing that but chrysler has been known to do dumber things before.

Thank you.

I show two numbers for the A999 rear planetary, 3681988 and 8124325....both are 28-tooth and should interchange.

May have found a source for a new one. Looking through fleabay I found pictures galore of A500 drums and other related parts in the area. Annulus gear inside the reverse drum is the same per the parts book and the drums look identical besides the sprag change that I am fairly confident the newer planetary (pn 4431438 all 3/4 speed 88-04) would retrofit into the older drum.