Mass Tank Sales Corp.was founded more than 50 years to make fuel oil tanks for walk-up apartment buildings in Boston. Over the years, it diversified into water tanks, a variety of fueled tanks, and even confectionery tanks that store unprocessed candy for sweets makers like Necco and Tootsie Roll. Now, Mass Tank has found a new line of business: wind power. The company is now making 20-story towers that support wind turbine blades.