A thief boosted a bakery truck on the Upper East Side Monday — but instead of selling the $60,000 ride, he dutifully made the regularly scheduled dropoffs along the driver’s route, police sources told The Post. Wearing only his underwear, David Bastar, 30, of Nanuet stole a Grimaldi’s Home of Bread truck — full of $8,000 worth of baguettes, whole-wheat rolls and loaves of sourdough — on Second Avenue near East 99th Street around 3 a.m., police said. He then followed instructions mapped out on a piece of paper on the front seat, parceling out the baked goods to at least three restaurants and stores, police and bakery workers said.