While the 396 was an available option in 1966 Impalas, the highest horsepower rating for that engine in a full-size Chevrolet that year was significantly lower than 425 hp. The most powerful 396 available was likely around 375 hp in the Corvette or some high-performance models, and even less in the full-size Impala line. A 4-door Impala would have had an even lower horsepower rating, probably in the 200-300hp range for a 396 if it were offered at all in that body style.
A 425 hp 396 in a 1966 Impala 4-door would almost certainly be a result of a later engine swap and significant modifications, not a factory configuration. Someone could have installed a 396 and heavily modified it to achieve that horsepower.
