I guess you have an existing Caesar II calc or similar for the 16" piping since springs have been designed. If not, model it. You will need to consider the extra weight from your valves and bypass, and how that affects the springs.

You'll also need to think about temperature and expansion stresses due to your bypass. When it's cold and the header hot there should be no thermal stress in the bypass or connections. With the whole system cold, the bypass will be stressed by header contraction. When all hot, the bypass will be additionally stressed. You could use equivalent temperatures to represent these relative changes. Be careful since thermal expansion is not exactly linear with temperature change.