GM rocked the industry for 1977 with smaller, lighter full-size cars that were no less roomy inside than the outsize gas-guzzlers of 1971-1976. Buick offered the "downsized" coupes and sedans under its LeSabre and Electra banners, with Estate Wagons a separate line as before. Engines shrank to match the cars, with a standard 350 V-8 and a new 403 at extra cost, Thus fir-door Limited was one of four Electras in the $6700-7200 range, The public responded and big-Buick sales picked up, Electras to