The 1977 Impala was almost a completely different car from the 1976 model.  It shrank both in steel and in mettle (pun intended).  The weight of the car dropped by just over 700 pounds, then length shrank 10.6 inches, it was 4 inches narrower, and the wheelbase was 5.5 inches shorter.  It wasn't exactly down to F-1 racer size, but it definitely was smaller than former models.

The engines also shrank.  The new standard was a 250-ci I6 rated at 110 horsepower. Available engines included a 305-ci V8 rated at 145 horsepower and a 350-ci V8 rated at 170 horsepower.