Here's the axle width info from the 1996 FSM (94/95 may differ slightly as far as ratio to application):

There are 3 different widths, 1566mm, 1598mm (both sedan), and 1642.5mm (wagon). This discussion is primarily about the sedan axles. I "believe" that the width dimension refers to the distance from axle flange to axle flange--without disc or drum thickness added on.

The 1566mm axle (61.65") is used on 8.5" axle applications only, with both disc and drum brakes, depending on the car (taxi = 11" drum with 2.93 or 3.23, police = disc with 3.08 or 3.23)

The 1598mm axle (62.91") is used on 7.625 & 8.5 applications, both disc & drum, 2.56, 2.93. 3.08 & 3.23. Impala SS is the ONLY 1598mm axle with rear disc brakes, and is the only wider axle that used 3.08 ratio.

The difference in width between the 2 sedan axles is 32mm (1.26"), which means that a wheel mounts "at least" 5/8" further outboard (on each side) on the Impala SS than a 9C1. I say "at least", because, in addition to the width difference in the axles, there will be additional spacing of the wheel either by the thickness of a brake drum or a brake disc. I don't have a brake drum to measure, but I suspect that the rear brake rotor "hat" may be slightly thicker than a drum. If anyone can provide a drum measurement, I'll be able to compare them.

That 5/8" is a lot of space to "close up" to get, for example, a 9C1 wearing Impala wheels, to look as broad-shouldered as the "real McCoy". It doesn't take very much change in the wheel/tire position at the fender edge to have a really dramatic effect on appearance. Spacers, anyone?

Buick sedan axles are all 1566mm, 8.5" ring gear, all drum brake. 2 ratios available in 96, 2.56 & 2.93. Wagons are same as Chevy.