how do I identify correct A body gas tank?

I don't know if I can help but I'll try . Two types of tanks : one has a single gas tank strap and the other has 2 straps . I don't think they are interchangeable . The tanks with 2 straps come in 2 sizes , 13 gallon and 16 gallon , I THINK . IN years gone by as now the capacity was determined by the average distance a tank would carry the car driving at a normal (ha) rate of speed . Usually 300-350 miles . There may be some fitment issues between cars with wheel bases of 108 vs. 111". Hope this helps