You spent some credibility getting to this point with your buyer or organizational leadership. Show the return on trust, a newly coined term like return on investment.
Ask teams to capture any positive quotes from interactions with customer stakeholders. Iteration demos are a great place for this. A side benefit is that development team members get direct feedback that traditionally they have not heard, except perhaps second hand. Team members really like to hear good things about all their hard work.
Summarize some of the choicest comments for sharing with leadership. Some may be good enough to go into customer testimonials for bidding your next project.
Third party feedback like this is more believable to leadership than your own talking points about your project. If your organization is still hierarchical, your next level leadership may share these quotes with the next level of leadership. If your organization has already flattened its structure, then the level of leadership hearing it first may be the VP level, and they may share that amongst each other. Some of this depends on the culture of collegiality versus competition among executive leaders.