How to design a different 1958 Mutual Defence Agreement

What would a 1958 Anglo-American Mutual Defence Agreement look like with a non-declined British Empire?

Build it from the altered goals and leverage of both sides; the historical agreement would not simply carry over.

The original agreement is the wrong template when the political and strategic setting has changed. The useful starting questions are what Britain and the United States each want, what each needs, and what each believes it can obtain. Those bargaining positions would produce a genuinely new agreement rather than a historical text with altered labels. The method is broadly useful for disciplined alternate-history analysis.