What chassis was used with the Chevrolet engine in 1991?
CREATE TABLE "indy_500_results" (
  "year" real,
  "chassis" text,
  "engine" text,
  "start" text,
  "finish" text,
  "team" text
);
SELECT "chassis" FROM "indy_500_results" WHERE "engine"='chevrolet' AND "year"=1991;