Late model rear end
I believe the Ferd Explorer has the 8.8" rear end and the width should be right for an A body. I imagine you'd have to change the spring perch locations to make it fit. These things come with a pretty impressive array of good gear ratios and a lot of them have the (posi) rear ends, for want of trying to remember the correct Ferd term for their suregrip. :) Another one that might work is the Jeep Cherokee diffy, although I haven't measured one for width.