BRAINTREE — Town officials are strongly opposed to a proposed electronic billboard in Quincy at the site of the Elevation apartments. The John Flatley Co. of Canton, which developed the apartment complex, wants to place a 14-by-48 foot billboard within a few feet of where a sign for the development now stands. The top of the sign would stand 35 feet above the ground alongside highway ramps for motorists heading into Boston on Route 3 or taking Interstate 93 towards Dedham. Residents across the highway in Braintree, which is at a lower elevation than the billboard site, fear that the billboard will be bright enough to shine into their homes, especially in the winter months when there are no leaves on the trees. They also have safety concerns about a billboard distracting drivers on busy highway ramps where traffic merges.