I always set those up with only a very small difference between valve-open and overflow though, never enough to put a big bending load on the tank or valve-body. In this case we have a completely submerged ball on the end of an arm (read lever), the mending moment will be large. Given that modern tanks are not made of galvanised steel, but polyethylene, and the valve bodies could be plastic too....
Isn't there some other kind of level-sensing valve?
R.