Once was a time when picking a good key lime pie was simple. A half-dozen well-known ingredients and a straightforward preparation added up to a consistent product that would always deliver that sweet-tart bite. You don't meddle with a good thing once it's perfected. But this is South Florida. People meddle. So you never know just what to expect from a particular establishment. Some places produce a sort of lime-flavor cheesecakelike confection, while others prepare a bright green yet bland sliver of custardy pie. Let's not even get into the variety of crusts and overdone whipped cream or meringue toppings. Ideally you want a pie that's a pale green and tart as a Granny Smith apple but with an underlying creamy sweetness that takes the edge off. It's a good finish to a meal of sweet, smoky barbecue ribs. Shorty's, a south-county throwback to the Fifties, delivers both of these well. Belly up to the long wooden benches and eat yourself silly. But save room for the bakery-delivered pie, $2.79 worth of simple delight. Open Sunday through Thursday 11:00 a.m. to 10:00 p.m., Friday and Saturday till 11:00 p.m.