I think I remember that sign from childhood. The OP is correct. The Atlantic gas station at left of frame (Atlantic Richfield Co. (ARCO)) is now an Irving gas station at Belmont and Natick St. The building just behind the sign is now D&B Auto Body. The image focal length is compressed because it was shot with a long lens. But the Citgo must have been right at the junction of Shrewsbury Street and Belmont, just across the street from what was then United Chevrolet (now U-Haul). The cars in the distance (coming down the hill) are probably stopped at the light at Plantation Street.