I'm not sure exactly what sort of tanks are being discussed since the original poster did not completely describe.
I've got near twenty of the 250 gallon cube-shape tanks that were originally used for molasses, cider, etc. Square shaped and originally had a steel cage around them so they can be stacked on top of each other. I see them all over the country. Every one of them that I have, has a male stub on the bottom of the tank that is 2 13/16" OD with 6 threads per inch. The shut-off valve that is usually screwed onto the tank has 2 7/8" female with 6 threads-per-inch that goes onto the tank.