The 400 was replaced for 1970 with the 455 cu in (7.0L) Buick V8, used in the GS 455. The base model V8 produced 350 hp (260 kW) and 510 ft·lbf (690 N·m) at 2800 rpm. In the optional Stage1 trim it produced 360 hp (268 kW) and 510 ft·lbf (691 Nm) at a low 2800 rpm. As with all American engines produced prior to the 1972 model year, these were SAE gross ratings, which are generally significantly higher than SAE net ratings and are indicative of what actual production engines produce in their "as installed" condition (with all engine accessories and full exhaust system in place). The fastest magazine test results from this period were obtained by MOTOR TREND Magazine, which managed to extract a 13.38 ET @ 105.5 MPH from their 3,810 pound GS Stage 1 coupe test car. Using Hale's Trap Speed formula, this result indicated actual "as installed" peak HP of approximately 360 SAE Net (ironically the same as its advertised Gross Figure, meaning this engine was very conservatively rated for that time period).
Source: Wikipedia, 2011