Corporate 10 bolt rear end
If you have a 1965-1976 Impala with a 12 bolt you can ignore this. However, if you have a ten bolt rear it will have 28 spline axles and an 8.2" dia. ring gear with a small dia. shaft pinion gear. It is stronger than a 7.5" late model ten bolt only marginally. The "Corporate" ten bolt rear end first appeared in 1972 and was designed to replace the twelve bolt rear end. It was a compromise between the larger stronger 8.785" dia ring gear that was used in the 12 bolt, with the same 30 spline axles used in the 12 bolt; compared to the 28 spline 10 bolt rear (some say they are not interchangeable, but if you grind a little off the end of a 12 bolt axle it will slip right into a Corporate 10 bolt rear end and allow the C-clip to be installed). The spline count determines the axles diameter with more splines equating to a larger diameter stronger axle (Ford 9" rears for example use a 31 spline axle, which is replaced in the aftermarket race ready rears with a 33 spline axle, Dana 60's have 35 spline axles).
The problem you will encounter with installing a late model corporate 10 bolt into a 1965-1976 B-body is that it is not a bolt in swap. You will have to carefully remove your old mounting brackets (or buy new ones from Currie or Moser) and then weld them onto the Corporate rear end after you cut off the old brackets and grind down the welds. This is because though both production runs used a four link with coil springs the 1965-1976 cars had parallel control arms and used a panhard bar to center the rear end, while the 1977-1996 cars used two offset upper control arms (set at a 45° angle without a panhard bar) to center the rear end. It is this 45° angle that makes it unadvisable to use polyurethane bushings in the upper control arms in 1977-'96 cars because they will bind when the suspension travels due to the angle and the arc it must traverse causes interference.
The Corporate 10 bolt rear end was used under all 1973-1996 9C1 cars and station wagons, large sedans from Cadillac and Buick and Pontiac. It can be found in pick up trucks as well, but it will be the wrong length unless you are planing to run custom wheels, or to remove the ends of the housing to get rid of the C-clips by means of Ford large bearing kits. There is an odd ball variant of the ten bolt used under Olds cars that have 12 bolts on the inspection cover and press on axle bearings (no C-clips). It is an orphan with out aftermarket parts to change gears ratios or to swap out for stronger aftermarket axles. It was found under '68-'70 Olds only cars and has an 8.3" ring gear.