One for starters, there is no such thing as a Chrysler 8.75 489 391 "posi"...........
It's called a Sure Grip. Posi is a GM term. Yeah, I'm gonna have to be that "guy"
a Cone style sure grip in unknown working condition, if you can get 400 out of just the center chuck, you'd be doing good. Lately, the cone style's are going for around 300-400 max. Complete Rear end, you're looking maybe 4 to 6 hundred tops.