GM's Chevrolet Motor Div. andreport May sales at blistering levels. With Tahoe running 60% ahead of the prior year and Cavalier up 46%, Chevy had its best May since 1989 with 260,557 combined car (120,341) and truck (140,216) sales. Chrysler had its best month ever: 245,289, beating June 1995's 236,289. Meantime, Pontiac sets the base prices for the all-new '97 Grand Prix starting at $18,579 for the SE sedan, rising to $19,359 for a GT Coupe and $20,359 for the GT sedan. Last year's base prices were $17,629 for a sedan and $18,899 for a coupe, but neither had the 3.8L supercharged engine available on the '97. Also notable: says it expects to produce its 250 millionth vehicle during 1996.