Did the Impala have a F40 Heavy Duty Suspension option? The F40 RPO code was offered on Chevelles, Camaros and Novas, usually as part of the police package, but could be ordered by anyone. It included stiffer springs and shocks that damped oscillations faster for better handling.
I am thinking with the 283 four arm Impala that might explain it. Other wise the four control arms and a 12 bolt in the Impala was reserved for a motor with a factory rated 300 horse or more (usually but not restricted to big blocks), or a police car (the 9C1 RPO group).
A police car was the basis for the Impala Super Sport's return in 1994; and was always a better package than the Super Sport option offered civilians, as it had more HD features. In the Chevelle, Camaro, and Nova models you could order a SS that included upgraded springs and shocks with the F41 RPO code as part of the SS option, and include a F40 RPO on top of that, to get a full race suspension from the factory ready to run TransAm.