$200.00 for a 10b posi (G80) around here.
'90's Impala's, especially the wagons, came with eaton style posi units. That's the one to have! And they fit the truck 10b housing. They are rebuildable, including the ability to adjust the pressure on the clutches via the different springs available.
Just make sure that the spline count is the same between the eaton & the axles. Look for the 'posi fluid only' tag on the housing, attached to one of the cover bolts. Those do go missing, though, so that's not always a reliable way to identify them. Checking the build sheet in the glove box is another way.
I wouldn't waste the time, energy & money to install a G80. They're called a gov-bomb for a reason!