The '77-'90 Caprices and Impalas (4th-Gen B-Body) have 15" x 7" wheels. I'm not sure on the backspacing, either 4.5" or 4". Most cars (retail sedans & coupes) have the 5 x 4.75" bolt pattern. The wagons and 9C1/9C6 cars have the 5 x 5" bolt pattern, this is due to the HD brakes found on them.
For the '91-'96 cars (all sedans & wagons), all wheels are the 5 x 5" bolt pattern.
I have seen 14" wheels on 4th-Gen B-Bodies with the 5 x 4.75" pattern. It ain't pretty though.