He might be fine NOW. And that is fine, i have no problem with people having a pet and upgrading tanks as it gets larger. The 180 gallons is actually pretty darn close so you would be pretty much set. The 24" width is what is really important. They just do better with that extra foot or two of length is all. IMO
you would be pretty much set, it is that extra foot or 2 are just a little bit better. You also have to remember that stingrays are surprisingly very active. What is his length in diameter would you say and his length? Also if you could snap a pic of him unburied.