Time's up for Buick's Roadmaster, Cadillac's Fleetwood and Cheverlot's Caprice and Impala SS.Corp. will halt production of the three biggies this summer, switching production to trucks or sport/ utility vehicles (SUVs) at the Arlington, TX., assembly plant where the boulevard boats are built. Now the idea is to keep fleet buyers - cabbies and cops - who love the durability, reliability and ride of large rear-wheel drive cars. Ongoing discussions at GM focus on three scenarios: convert Chevrolet's Lumina to all-wheel drive, source a rear-wheel drive car from Opel AG and/or pitch GM's re-engineered '97 minivan to cab companies. It'll feature a left-side passenger door, a must in cities laced with one-way streets. GM wants to keep cabbies and cops happy, so it needs a low-maintenance, rear-wheel-drive vehicle that can take a pothole and keep on rolling. Motor Co. will keep its full-size, rear-drive Crown Vic a few more years and likely snap up some business abandoned by GM. As for the big Caddie, look for a high-end Fleetwood version of Deville when that model gets revamped for '97.