I'm not sure I follow exactly what your question is, but maybe this will answer it. The way an expansion tank is installed is with a tee in the cold water line between the PRV and water heater. The tank needs to be supported well, but can be vertical (hanging down) or horizontal. In other words, there is quite a bit of leeway on just where it ends up. From the tee in the water line, there is a copper fitting that comes with the tank that makes the transition, one side screws into the tank and the other side is designed to go into a 1/2" tee. If your tee is larger, then you have to reduce the portion of the tee that goes to the tank. You can use additional 1/2" pipe to position the tank as described above. I would also recommend a ball valve between the tank and the tee to isolate the tank if needed to install a new tank someday.