CREATE TABLE table_33406 ("Year" FLOAT, "Chassis" TEXT, "Engine" TEXT, "Start" TEXT, "Finish" TEXT, "Team" TEXT)

What chassis was used with the Chevrolet engine in 1991?
SELECT "Chassis" FROM table_33406 WHERE "Engine" = 'chevrolet' AND "Year" = '1991'
SELECT "Chassis" FROM "table_33406" WHERE "Engine" = 'chevrolet' AND "Year" = '1991'