I think what Jerry meant was that if the water has to come into the tank from the top, put a 90 degree adapter from the top down into the tank and the outlet will be horizontal, then you can install that float valve.
When you mentioned that the water has to come into the tank from the top, that is Jerry's suggestion to make it work from coming into the top of the tank down.
I don't think Jerry meant from side to side, just trying to adapt it to work from the top down so it would work horizontally with the adapter.
It is designed to operate up/down so if you can drill a hole in the side of the tank near the top, you could install it there, just make sure you have room for the float to reach to top of travel so it will shut off, otherwise the valve will never shut off.
Hope this helps.