Ok let's see if I got this. You want to put the 8.5 10bolt into your Impala? The kicker is that the B body upper arms are on the centre section and with the 8.5 you would need to add fabricated mounts that are lower and further apart than on the stock diff.
You'll have to look but there should be some after market 4-link kits that may you may be able to adapt to your application.
