According to Wikipedia, the top engine for '76 was the 400:
In its third and final season, the 1976 Laguna Type S-3 again featured a sloped, body-color urethane front end and quarter-window louvers, but with new rear styling and horizontal taillights shared with other Chevelles. Lagunas shared their round-gauge instrument panel with the personal-luxury Monte Carlo; Malibu models made do with a more conventional dashboard and a linear-readout speedometer. The standard engine was a new 140-horsepower 305 2-barrel V8. Optional engines included the 145-horsepower 350 two-barrel V8 and 175-horsepower 400 four-barrel V8. Aero styling had helped make the Laguna popular with NASCAR drivers. Laguna S3 was outsold by the lower priced Malibu Classic coupe and the Monte Carlo. Production of the Laguna edged up to 9,100 cars as the base price went to $4,621.
NASCAR